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Vol.  XXX 
No.  2 


Whole  No.  135 
1921 

/  v 


PSYCHOLOGICAL  REVIEW  PUBLICATIONS 


THE 

Psychological  Monographs 


JUL  27  1921 
^S!CAJ.  $9^? 


EDITED  BY 


JAMES  ROWLAND  ANGELL,  522  Fifth  Avenue,  New  York. 
HOWARD  C.  WARREN,  Princeton  University  ( Review ) 

JOHN  B.  WATSON,  New  York  (/.  of  Exp.  Psych.) 

SHEPHERD  I.  FRANZ,  Govt.  Hosp.  for  Insane  ( Bulletin )  and 


MADISON  BENTLEY,  University  of  Illinois  {Index) 


STUDIES  FROM  THE  PSYCHOLOGICAL  LABORA¬ 
TORY  OF  THE  UNIVERSITY  OF  CHICAGO 


The  Influence  of  the  Time  Interval 
Upon  the  Rate  of  Learning 
in  the  White  Rat 


BY 


JOSEPH  U.  YARBROUGH,  Ph.D. 


Professor  of  Psychology,  Southern  Methodist  University 


PSYCHOLOGICAL  REVIEW  COMPANY 
PRINCETON,  N.  J. 
and  LANCASTER,  PA. 


Agents:  G.  E.  STECHERT  &  CO.,  London  (2  Star  Yard,  Carey  St.,  W.  C.) 

Paris  (16  rue  de  Cond6) 


Acknowledgments 


The  experiments  herein  reported  were  carried  out  under  the 
direction  of  Dr.  Harvey  A.  Carr  of  The  University  of  Chicago 
who  has  been  unsparing  in  suggestion,  supervision,  and  criticism. 
To  him  I  wish  to  express  my  sincerest  gratitude  for  his  un¬ 
wearying  kindness  and  interest. 

My  thanks  are  due  Professor  James  R.  Angell,  Dr.  W.  S. 
Hunter,  and  Dr.  Clarence  S.  Yoakum  for  their  thorough  and 
stimulating  instruction.  My  initial  interest  in  Animal  Psychol¬ 
ogy  is  due  to  my  contact  with  the  enthusiasm  and  scientific 
ability  of  Dr.  Hunter. 

For  helpful  suggestions  I  am  indebted  in  particular  to  the 
work  of  Wylie;  in  general,  to  those  other  investigators  whose 
work  is  cited  in  this  report. 

Acknowledgment  is  also  due  Mr.  J.  Hall  Sheppard,  a  graduate 
student  in  the  University  of  Texas,  for  assistance  in  the  early 
training  of  some  of  the  animals,  and  to  Mrs.  Yarbrough  for 
assistance  in  preparing  the  manuscript. 

Jos.  U.  Yarbrough. 

University  of  Chicago, 

Feb.  25,  1920. 


TABLE  OF  CONTENTS 

PAGE 

I.  Introduction  .  i 

II.  Historical  .  3 

III.  Apparatus  and  Method .  13 

IV.  Experimental  Results  .  19 

1.  The  Influence  of  the  Time  Interval .  19 

A.  Learning  the  association  in  the  forward  direc¬ 

tion  .  19 

a)  Association  of  sound  with  pain .  19 

b)  Association  of  light  with  sound .  24 

B.  Learning  the  association  in  the  backward  di¬ 

rection  .  30 

2.  Simultaneous  vs.  Successive  Presentation .  35 

3.  Learning  the  Negative  Response  to  Pain .  39 

V.  Interpretation  of  Results  .  44 

VI.  Summary  and  Conclusion .  50 


I. 

Introduction 

The  experiments  reported  in  this  monograph1  are  concerned 
with  three  closely  related  problems  in  the  field  of  animal  psychol¬ 
ogy  :  i )  The  influence  of  the  time  interval  upon  the  rate  of  learn¬ 
ing.  The  animals  were  divided  into  groups,  and  the  terms  X 
and  Y  were  presented  in  a  temporal  relation  to  each  group.  The 
comparative  number  of  trials  necessary  to  establish  a  given 
strength  of  functional  connection  constitutes  the  measure  of  the 
relative  efficacy  of  the  several  modes  of  presentation.  2)  The 
readiness  with  which  a  given  temporal  association  will  function 
backward,  and  the  most  effective  time  interval  for  this  function¬ 
ing.  The  terms  X  and  Y  were  here  presented  in  the  order  Y-X, 
but  in  the  test  experiments  they  were  required  to  function  in  the 
normal  order  X-Y.  3)  The  claims  of  the  rival  thories  of  simul¬ 
taneous  and  successive  presentation  in  relation  to  the  rate  of 
learning.  The  animals  were  required  to  associate  two  factors, 
X  and  Y.  To  one  group  the  terms  were  presented  simul¬ 
taneously;  to  the  others,  successively. 

The  nature  of  the  present  experiments  may  be  further  set 
forth  by  additional  explanation  of  the  problems  under  considera¬ 
tion.  The  various  statements  made  in  the  modern  texts  on  gen¬ 
eral  psychology  indicate  that  simultaneous  or  successive  presen¬ 
tation  is  regarded  as  an  essential  condition  for  the  formation  of 
an  effective  association.  And  by  successive  is  meant  immediate 
succession;  i.e.,  the  two  experiences  must  be  in  temporal  contact 
with  each  other.  Our  aim  is  to  separate  these  terms  by  a  certain 
interval  and  observe  its  influence  upon  the  rate  of  learning  the 
association  when  the  terms  are  presented  in  the  forward  and 
in  the  reverse  order.  Those  who  hold  to  the  theory  of  simul- 

1  This  research  was  begun  in  the  Psychology  Laboratory  of  The  Uni¬ 
versity  of  Texas  in  October,  1918,  and  continued  until  the  following  June, 
at  which  time  the  work  was  transferred  to  the  Psychology  Laboratory  of 
The  University  of  Chicago. 


2 


JOSEPH  U.  YARBROUGH 


taneous  association  would  have  us  believe  that  association  can¬ 
not  be  formed  unless  the  facts  or  objects  associated  be  ex¬ 
perienced  simultaneously.  They  recognize  the  fact  that  in  every¬ 
day  life  things  are  associated  which  succeed  each  other  in  time, 
but  they  account  for  this  on  the  assumption  that  “with  the  ces¬ 
sation  of  the  actual  mental  experience  the  nervous  excitation  is 
not  abruptly  finished  but  continues,  gradually  dying  away.”2 
The  second  factor  of  the  association,  then,  coincides  in  time  with 
this  “gradually-dying-away”  nervous  excitation  of  the  first,  and 
it  is  due  to  this  simultaneity  that  the  association  of  the  two  fac¬ 
tors  is  formed.  The  rival  theory  of  successive  association,  based 
upon  the  hypothesis  that  the  range  of  consciousness  is  not  wide 
enough  to  attend  to  two  things  at  once,  denies  the  possibility 
of  association  between  two  factors  simultaneously  presented. 
Those  who  hold  this  theory  say  that  although  the  two  factors  are 
apparently  presented  simultaneously  they  are  in  reality  successive 
experiences,  since  the  attention  is  constantly  vacillating  between 
the  two.  If  successive  association  is  correctly  explained  by  the 
simultaneity  of  the  succeeding  experience  with  the  after-phase  of 
the  preceding  one,  the  data  here  presented  show  to  a  certain  ex¬ 
tent  the  duration  of  this  “akoluthic”  phase  and  its  rate  of  dimi¬ 
nution. 

Although  our  prime  interest  is  in  the  first  of  the  problems 
listed  above,  it  was  found  both  practical  and  profitable  to  gather 
data  on  the  second  and  third.  Very  little  work  on  the  main 
problem  has  been  reported  either  in  the  field  of  animal  or  of 
human  psychology,  hence  it  is  on  this  point  that  these  experi¬ 
ments  will  probably  have  their  greatest  significance.  The  second, 
likewise,  has  received  scanty  attention  as  yet.  The  third  problem, 
on  the  other  hand,  has  been  the  subject  of  considerable  experi¬ 
mentation,  although  but  little  of  this  investigation  has  been  in 
the  field  of  animal  psychology.  It  is  our  purpose  to  present  data 
upon  these  timely  problems,  with  especial  emphasis  upon  the 
question  of  the  influence  of  the  time  interval  in  successive  asso¬ 
ciation  upon  the  rate  of  learning  in  both  its  backward  and  its 
forward  directions. 

2Semon,  Richard,  “Die  Mnene,”  2te  Auflage,  Leipzig,  1908. 


II. 

Historical  Section 

The  purposes  of  the  present  historical  section  are:  i)  To  bring 
together  for  critical  consideration  all  the  data  reported  up  to  the 
present  time  on  the  rate  of  learning  in  successive  association; 
2)  To  present  a  brief  critical  review  of  the  experimental  litera¬ 
ture  upon  the  subject  of  simultaneous  and  successive  presentation 
in  association;  3)  To  examine  all  data  now  published  on  the 
strength  of  function  of  association  in  the  backward  as  com¬ 
pared  to  the  forward  direction;  4)  To  show  that  the  evidence 
is  inconclusive  because  of  its  scantiness  and,  in  some  cases,  its 
general  character.  Due  to  the  fact  that  in  some  of  the  experi¬ 
ments  reviewed  more  than  one  of  the  above-named  issues  is 
discussed,  it  was  deemed  expedient  to  present  the  full  report  in 
each  case  rather  than  to  attempt  presentation  by  topics.  Crit¬ 
icism  and  discussion  have  in  some  cases  been  deferred  to  the 
Theoretical  Section. 

Bigham3  (1894)  was  among  the  first  to  investigate  these 
questions.  For  subjects  he  used  five  college  students  of  an 
average  age  of  24;  for  material,  series  of  varied  numbers  and 
colors  (separate  or  in  mixed  series)  mounted  on  cardboard  3F2 
cm.  in  size.  Five  simultaneous  series  were  used  and  five  suc¬ 
cessive.  In  every  respect  the  series  were  alike  save  in  the  manner 
of  presentation.  The  length  of  the  series  consisted  of  either  10  or 
20  presentations.  A  40  second  period  was  allowed  for  each 
series,  20  simultaneous  presentations  being  made  in  this  period, 
and,  in  the  successive  series,  two  seconds  being  allowed  for  each 
of  the  20  cards.  By  this  method  the  exposure  time  was  two 
seconds,  whether  the  cards  were  presented  simultaneously  or  in 
succession.  The  subjects  were  asked  to  use  no  memory  aids  in 
the  learning.  After  a  given  series  had  had  its  due  presentation 
the  subject  immediately  arranged  his  colors  or  numbers  in  the 

3  Bigham,  J.,  Psychol.  Rev.,  1894,  Vol  I,  pp.  34-38. 


4 


JOSEPH  U.  YARBROUGH 


order  of  the  presented  series  as  it  was  recollected.  He  saw  the 
series  but  once.  Obviously,  only  the  visible  series  gave  a  basis 
for  comparison,  and  in  these  Bigham  found  that  for  the  simul¬ 
taneous  series  the  percentage  of  error  for  the  five  subjects  ran: 

13.6,  9.8,  15.3,  17.9,  and  15.  In  the  successive  series  the  cor¬ 
responding  percentages  of  error  were  18.5,  23.3,  16.6,  24.1,  and 

21.6.  The  averages  for  the  two  series  were  14.3  percent  and 
20.8  percent  for  the  simultaneous  and  the  successive,  respectively. 
Hence,  he  concludes:  “With  each  observer  the  memory  was 
stronger  for  the  simultaneous  than  for  the  successive  presenta¬ 
tions.” 

Bergstrom4  (1907)  performed  a  series  of  experiments  pat¬ 
terned  after  the  work  of  Ebbinghaus.  Three  subjects  were  each 
required  to  memorize  12  lists  of  nonsense  syllables,  12  syllables 
to  the  list,  for  each  of  four  days.  Each  syllable  was  exposed  to 
view  for  a  period  of  82<r,  and  a  given  time  interval  was  allowed 
to  elapse  between  the  successive  exposures  of  the  syllables.  The 
syllables  were  to  be  pronounced  but  once  at  the  time  of  exposure, 
and  after  four  repetitions  of  a  list  a  recall  test  was  given.  In 
this  test  a  written  reproduction  was  made  and  the  amount  memo¬ 
rized  was  gauged  by  the  number  of  errors  made.  The  intervals 
between  the  exposures  were  3020-,  68 60,  and  1454^,  and  the 
average  number  of  errors  was,  respectively,  10.3,  8.9,  and  7.5. 
By  increasing  the  interval  between  exposures  the  number  of  er¬ 
rors  was  decreased.  This  fact  is  at  variance  with  the  assump¬ 
tion  that  a  nervous  impression  begins  to  fade  soon  after  it  is 
received  and  gradually  dies  away,  and  must  be  explained  by  the 
greater  strength  either  from  the  clearer  original  impressions  or 
perhaps  by  other  factors  which  are  favored  by  a  longer  time 
interval. 

Again,  30  subjects  were  used  by  this  same  experimenter,  and 
lists  of  10  words  and  lists  of  10  letters,  presented  orally  at  a 
metronome-controlled  rate,  were  used  as  material.  The  time 
intervals  between  presentations  here  were  .5,  1,  and  2  seconds. 
One  presentation  was  made  for  each  list,  and  the  errors  made  in 

4  Bergstrom,  J.  A.,  Amcr .  Journ.  Psychol.,  Vol.  XVIII,  pp.  206-238. 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


5 


immediate  written  reproduction  again  tested  the  amount  memo¬ 
rized.  For  the  three  intervals  the  average  percentages  of  error 
were,  respectively,  51.12,  36.52,  and  23.9  for  the  word  lists,  and 
44.09,  52.65,  and  38.44  for  the  letter  lists.  In  both,  the  amount 
learned  increased  with  the  lengthening  of  the  interval.  Berg¬ 
strom  concludes :  “The  effect  of  increasing  the  interval  is  greatly 
to  decrease  the  number  of  errors.  .  .  .  Indications  are  that  an 
interval  longer  than  any  employed  would  have  still  further  re¬ 
duced  the  errors.” 

Wohlgemuth5  (1914)  reports  an  experiment  the  purpose  of 
which,  as  he  states  it,  is  1)  “to  decide  by  experiment,  if  possible, 
between  the  claims  of  the  rival  theories  of  Simultaneous  and 
Successive  Association,”  and  2)  “to  investigate  the  influence  of 
the  closeness  of  connection  between  the  members  presented  simul¬ 
taneously.”  He  used  eight  subjects,  whose  age  and  qualifica¬ 
tions  were  undefined.  To  them  he  gave  a  series  of  tests  con¬ 
sisting  of  pairs  composed  of  a  colored  field  (cardboard)  fol¬ 
lowed  by  a  black  shape  mounted  on  a  white  ground,  or  vice- 
versa.  The  exposure  was  timed  by  a  Muller  memory  apparatus, 
a  certain  definite  period  of  exposure  thus  being  given  each  card. 
Obviously,  under  such  conditions  the  simultaneous  pair  would 
have  just  the  same  period  of  exposure  as  each  member  of  the 
successive  pair.  He  found  the  successive  mode  of  presentation 
superior  to  the  simultaneous,  but  accounted  for  the  difference 
on  the  basis  of  the  unequal  exposure  time  for  each  pair  of  stimuli. 
To  overcome  this  defect  two  groups  were  formed:  in  the  first, 
only  half  the  number  of  exposures  was  given  in  the  successive 
series,  while  each  member  of  the  pair  was  exposed  for  the  same 
length  of  time  as  the  pair  of  simultaneous  stimuli — i.e.,  the  rate 
of  the  memory  apparatus  remained  the  same;  in  the  second,  the 
same  number  of  exposures  was  given  in  both  series  and  the  same 
time  given  each  pair,  but  only  half  time  was  given  each  member 
of  the  successive  pairs — i.e.,  the  apparatus  was  set  to  work  twice 
as  rapidly  in  the  successive  series  as  in  the  simultaneous.  Wohl¬ 
gemuth  finds  that  in  the  first  group  the  average  scores  were  35.5 

5  Wohlgemuth,  A,  “Simultaneous  and  Successive  Association,”  British 
Journ.  of  Psychol.,  VII,  1914-15,  pp.  434-452. 


6 


JOSEPH  U.  YARBROUGH 


for  the  simultaneous  and  33.2  for  the  successive;  in  the  second 
group,  36.8  and  34.4,  respectively.  Hence,  he  concludes :  1 ) 
“The  simultaneous  experience  is  more  favorable  for  the  learn¬ 
ing  of  pairs  than  the  successive  experience.”  2)  “All  associations 
are  due  to  simultaneity  or  simultaneity  of  the  succeeding  ex¬ 
perience  with  the  akoluthic  phase  of  the  preceding  experience.” 

On  the  whole,  Wohlgemuth’s  article  is  far  from  convincing. 
It  seems  that  his  evidence  is  insufficient  to  warrant  such  positive 
statements,  and  one  is  inclined  to  agree  with  Froeberg0  in  his 
criticism  of  the  work.  Froeberg  says  that  Wohlgemuth’s  method 
of  presentation  must  be  rejected,  but  that  even  if  this  might  be 
admitted  “no  safe  conclusion  could  be  drawn  from  the  results 
because  the  differences  in  favor  of  simultaneity  are  in  every  case 
less  than  the  P.  E.,  and  in  five  cases  out  of  sixteen  they  are 
negative.”  Froeberg  calls  his  own  investigation  an  “attempt 
to  repeat  Wohlgemuth's  experiment  with  the  objectionable 
features  removed.”  He  used  psychology  students  for  subjects, 
and  had  them  memorize  series  of  five  pairs  of  syllables.  Each 
term  of  the  pairs  was  exposed  for  a  period  of  1/3  of  a  second, 
and  the  time  between  the  terms  varied  from  o  to  5  seconds.  The 
interval  between  the  successive  pairs  was  a  constant.  The  recall 
came  10  seconds  after  the  last  presentation  (each  series  having 
been  presented  twice),  and  was  conducted  by  presenting  the 
stimulus  syllables  in  a  new  order  and  requiring  the  subjects  to 
supply  the  missing  member  of  the  pair.  As  usual,  all  memory 
devices  were  barred  during  the  learning.  The  average  percent¬ 
ages  of  correct  response  for  the  group  of  seven  subjects  were 
61,  49,  45,  48,  51,  49,  and  49  for  the  simultaneous  and  con¬ 
tinuous  presentations  and  the  intervals  of  1,  2,  3,  4,  and  5  sec¬ 
onds,  respectively' — a  finding  which  would  indicate  that  the  dif¬ 
ference  between  the  simultaneous  and  successive  series  is  small, 
and  that  in  successive  presentation  the  length  of  the  time  interval 
is  not  effective.  The  simultaneous  exposure  yielded  better  re¬ 
sults  in  every  case,  however,  and  the  average  difference  of  12 
percent  in  its  favor  seems  significant.  Froeberg  does  not  think 

6  Froeberg,  S.,  Psychol.  Rev.,  1918,  Vol.  XXV,  pp.  156-163. 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


7 


that  this  slight  superiority  of  simultaneous  presentation  neces¬ 
sarily  proves  the  theory  of  simultaneity,  but  would  explain  it  by 
a  “tendency  to  articulate  and  combine  the  simultaneous  syllables 
into  a  single  word.”  From  the  introspection  of  his  subjects  he 
found  “essential  agreement  on  three  points :  i )  that  simul¬ 
taneous  exposure  taxes  the  attention  more  than  successive, 
2)  that  there  is  a  persistent  tendency  toward  articulation  in  the 
simultaneous  series,  and  3)  that  when  in  successive  association 
there  was  an  appreciable  interval  between  the  stimuli,  the  first- 
would  remain  in  consciousness,  though  with  varying  degrees  of 
intensity,  until  the  second  arrived.” 

In  order  to  eliminate  this  persisting  memory  image  during 
the  interval  between  the  two  stimuli  in  successive  presentation 
the  experiment  was  repeated  with  a  group  of  four  subjects  and 
with  the  added  condition  that  the  subjects  should  read  numerals 
presented  to  them  during  the  intervals  between  the  terms  of  the 
pairs,  a  device  calculated  to  prevent  memory  survival  of  the 
first  term  during  the  interval  separating  it  from  the  second.  In 
this  test  the  average  percentages  of  right  response  for  the  simul¬ 
taneous  and  continuous  presentations  and  the  time  intervals  from 
one  to  five  seconds  were  54,  45,  34,  36,  40,  24,  and  22 — i.  e.,  the 
rate  of  learning  decreased  with  an  increase  in  the  length  of  the 
interval  when  a  new  activity  was  interpolated.  This  decrease  in 
the  rate  of  learning  is  distinct  though  irregular.  From  these 
data  and  the  statement  by  each  of  his  subjects  that  “the  reading 
of  the  numerals  effectively  and  completely  obliterated  the  memory 
image  of  the  preceding  syllable,”  he  concludes  that  association 
may  still  be  formed  between  two  experiences  where  the  first  has 
already  passed  out  of  consciousness  when  the  second  one  appears. 
This  is  in  contrast  with  the  findings  of  Bergstrom. 

Since  Wohlgemuth  had  rejected  nonsense  syllables  as  improper 
material  for  the  study  of  association,  Froeberg  formed  another 
group  of  subjects  to  which  he  presented  colors  and  letters  of  the 
alphabet.  Other  details  were  the  same  as  in  the  previous  ex¬ 
periments.  He  found  under  these  conditions  that  the  successive 
was  superior  to  the  simultaneous  mode  of  presentation.  This 


8 


JOSEPH  U.  YARBROUGH 


finding  in  the  data  of  the  first  group  led  him  to  the  conclusion 
that  when  the  material  used  is  such  that  it  forms  an  organic  unit 
or  can  be  attended  as  a  unit,  simultaneous  presentation  is  prefer¬ 
able;  when  such  that  this  is  not  possible,  successive  presentation 
is  the  better. 

Chamberlain7  sought  to  ascertain  the  effect  upon  the  power  of 
recall  when  a  number  of  objects  were  displayed  i)  singly  and 
2)  three  together.  For  subjects  he  used  60  pupils  each  from  the 
third,  fifth  and  eighth  grades.  By  means  of  a  circular  disc  of 
wood  certain  objects  were  exposed  in  fixed  order  for  a  given 
length  of  time.  In  the  first  tests  these  objects  were  exposed 
three  at  a  time;  in  the  second,  one  at  a  time.  The  two  modes  of 
display  were  never  used  with  the  same  subject.  One  minute  was 
allowed  for  the  observation  of  15  objects.  In  the  first  tests 
each  object  was  exposed  for  nine  seconds,  while  in  the  second 
series  three  seconds  were  allowed  each  object.  It  is  obvious, 
then,  that  although  one  minute  was  given  in  both  cases  for  the 
observation  of  the  fifteen  objects,  the  exposure  time  per  object 
was  unequal.  The  immediate  recall  was  tested  in  every  instance 
by  having  the  pupil  recall  at  the  close  of  the  experiment  all  the 
objects  he  could  remember.  These  objects  were  listed  by  the 
experimenter  in  the  order  recalled.  The  subjects  were  also  pro¬ 
vided  with  circular  sheets  of  paper  and  asked  to  arrange  the 
objects  thereon  as  remembered  from  the  circular  disc.  The 
author  concludes  that  both  recall  and  arrangement  of  objects  are 
stronger  when  the  objects  are  seen  three  at  a  time  than  when 
shown  singly,  a  conclusion  scarcely  warranted  by  the  insignificant 
percentage  of  difference  between  the  two  modes  of  presentation. 
The  average  number  recalled  by  the  three  grades  when  the  ob¬ 
jects  were  presented  singly  was  9;  when  presented  three  at  a  time, 
9-37- 

In  the  human  field  investigations  on  the  direction  of  associa¬ 
tion  have  been  made  by  Ebbinghaus,  Muller  and  Schumann, 
Muller  and  Pilzecker,  and  Wohlgemuth.  In  each  investigation 

7  Chamberlain,  A.  H.,  “A  Memory  Test  with  School  Children/’  Psychol. 
Rev.,  1915,  XXII,  71-76. 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


9 


memory  material  was  used.  Ebbinghaus8  employed  the  relearn¬ 
ing  method  and  found  a  strong  tendency  for  an  association  to 
function  in  the  backward  direction,  but  its  strength  was  never 
equal  to  that  of  the  forward  tendency.  Muller  and  Schumann9 
using  practically  the  same  method,  confirmed  these  results. 
Later,  Muller  and  Pilzecker,10  using  the  method  of  recall,  found 
that  when  pairs  of  syllables  had  been  learned  as  trochees  and 
the  first  syllable  was  shown,  in  50  percent  of  the  cases  the  second 
syllable  was  reproduced,  while  when  the  second  syllable  was 
shown  the  first  member  of  the  pair  was  recalled  in  only  38 
percent  of  the  answers.  Here  a  backward  tendency  is  shown, 
but  it  is  of  less  strength  than  that  of  the  forward  direction. 

Working  in  the  Psychological  Laboratory  of  the  University 
of  London  Wohlgemuth11  experimented  rather  extensively,  vary¬ 
ing  both  method  and  material.  He  had  come  to  doubt  the  uni¬ 
versal  application  of  the  results  obtained  by  the  use  of  nonsense 
syllables  in  the  study  of  memory,  and  to  verify  the  results  he 
used  colors  and  diagrams.  To  make  a  further  check  on  his  find¬ 
ings  he  used  syllables  in  exactly  the  same  way  as  he  did  colors 
and  diagrams.  With  five  of  his  subjects  the  forward  tendency 
was  the  stronger,  and  with  two  the  backward  and  forward  ten¬ 
dencies  were  about  equal  in  strength.  He  concludes  that  the 
forward  tendency  is  the  stronger  when  articulation  is  used  in 
memorizing,  but  that  the  two  tendencies  are  of  equal  strength 
when  articulation  is  prevented.  Just  in  proportion  as  the  motor 
element  approaches  complete  elimination  do  the  results  indicate 
an  equal  functioning  strength  in  both  directions. 

In  the  field  of  Animal  Psychology  there  is,  as  we  have  said 
before,  very  little  work  reported  upon  the  problem.  Carr12  is 

8  Ebbinghaus,  “Uber  das  Gedachtniss,”  Leipzig,  1885. 

9Miiller  and  Schumann,  Experimented  Beitrage  zur  Untersuchung  des 
Gedachtnisses,”  Ztsch.  f.  Psychol,  u.  Physiol.  Sinnesory,  1894,  VI. 

10  Muller,  C.  E.,  and  Pilzecker,  A.,  “Experimented  Beitrage  zur  Lehre 
von  Gedachtniss,”  Ztsch.  f.  Psychol.,  1900,  Ergiinzungsbd.  1. 

11  Wohlgemuth,  A.,  “On  Memory  and  the  Direction  of  Association,”  Brit. 
Journ.  of  Psychol.,  1912,  Vol.  V,  p.  447. 

12  Carr,  H.  A.,  “Length  of  Time  Interval  in  Successive  Association,” 
Psychol.  Rev.,  Vol.  XXVI,  pp.  335-353. 


10 


JOSEPH  U.  YARBROUGH 


the  chief  contributor.  He  reports  a  series  of  brief  but  suggestive 
experiments.  First,  he  reports  three  short  experiments,  all  bear¬ 
ing  upon  the  subject  of  the  influence  of  the  time  interval.  White 
rats  were  used  in  each  of  the  experiments,  and  the  same  problem 
box.  The  problem  for  the  rats  was  to  learn  to  choose  between 
two  pathways  to  a  food-box  in  a  given  temporal  sequence,  R,  L, 
R,  L,  etc.  In  case  of  incorrect  choice  the  entrance  to  the  food- 
box  was  barred.  Always  the  day's  work  began  with  pathway  R. 
Between  the  successive  runs  of  a  day’s  series  the  rats  were 
allowed  to  eat  for  a  definite  time  interval.  In  the  first  experi¬ 
ments  three  groups  of  eight  rats  each  were  used,  and  the  inter¬ 
vals  for  the  groups  were  15,  25,  and  35  seconds,  respectively. 
The  results  indicate  no  essential  differences  between  the  groups 
in  the  initial  stages  of  the  learning,  and,  indeed,  no  essential  dif¬ 
ferences  throughout  for  the  15  and  35  second  intervals.  The 
group  tested  on  25  seconds  interval  manifested  a  quite  rapid  rise 
from  the  1  oooth  to  the  1200th  trial.  This,  however,  is  an  in¬ 
conclusive  finding,  as  other  factors  than  the  time  interval  may 
have  influenced  the  result.  The  group  was  too  depleted,  unfor¬ 
tunately,  to  permit  of  further  continuation  of  the  experiment. 
In  a  second  experiment  three  groups,  six  rats  each,  were  tested 
for  intervals  of  5,  10,  and  15  seconds,  respectively.  In  this 
test  the  entrances  to  the  pathways  R  and  L  were  alternately 
blocked  so  that  errors  were  impossible.  Test  series  in  which  the 
pathways  were  left  open  were  interpolated  every  fifth  day. 
Thus  the  rats  were  forced  to  choose  between  the  pathways  and 
the  learning  was  measured  by  the  number  of  correct  choices 
made  in  these  test  series.  To  quote  the  results:  “The  group  dif¬ 
ference  is  not  very  significant,  but  the  results  indicate  that  10 
seconds  is  a  more  favorable  interval  than  15  seconds.”  In  brief, 
he  admits  that  his  findings  are  inconclusive  as  regards  the  in¬ 
fluence  of  the  time  interval  upon  learning,  but  states  that  they  do 
prove  that  “an  association  nexus  can  be  established  over  a  con¬ 
siderable  interval  of  time,”  and  that  they  make  possible  the  ac¬ 
ceptance  of  the  hypothesis  of  direct  connection  between  the 
factors  in  successive  association. 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


ii 


Carr  and  Freeman13  report  an  experiment  upon  the  problem  of 
simultaneity  vs.  succession,  and  upon  backward  vs.  forward  direc¬ 
tion  in  forming  an  association.  A  box  with  zigzag  runways  was 
used.  In  preliminary  tests  rats  were  taught  to  run  these  path¬ 
ways  in  order  to  obtain  food.  Afterwards  in  80  per  cent  of 
the  trials  the  door  from  the  chosen  pathway  into  the  food-box 
was  closed,  and  the  rats  were  forced  to  retrace  and  run  the  op¬ 
posite  pathway  in  order  to  reach  food.  At  this  stage  the  rats 
were  divided  into  three  groups  for  the  purpose  of  testing  the 
formation  of  an  association  between  an  auditory  stimulus,  buzzer, 
and  the  act  of  turning  in  response  to  the  closed  door.  With  one 
group  the  buzzer  was  sounded  just  as  the  rat  was  in  the  act  of 
turning — simultaneously  with  the  response  to  the  closed  door. 
With  another  the  sound  was  given  at  a  definite  point,  P,  ap¬ 
proximately  one  second  before  the  door  was  reached.  The  third 
group  was  given  the  sound  at  point  P  in  the  return  pathway 
approximately  one  second  after  the  rat  had  turned  around  from 
the  closed  door.  Test  series  wherein  the  sound  was  given  at  any 
point  within  the  pathway  were  used  to  test  the  formation  of  the 
association.  It  was  found  that  “successive  presentation  of  the 
two  terms  constitutes  a  much  more  favorable  method  for  their 
association  than  does  simultaneous  presentation.”  The  groups 
on  backward  association  showed  slight  evidence  of  learning  when 
the  experiment  closed.  These  results  are  extremely  suggestive, 
but  some  points  of  weakness  should  be  pointed  out:  i)  The 
time  interval  between  the  two  terms  to'  be  associated  was  not 
constant.  Those  who  are  acquainted  with  the  behavior  of  white 
rats  would  not  expect  them  to>  traverse  the  distance  from  a  point, 
P,  to  the  closed  door  at  the  same  speed  each  time.  2)  The 
buzzer  which  was  the  source  of  the  sound  stimulus  was  in  contact 
with  the  maze.  The  animal  may,  therefore,  have  been  responding 
to  a  kinaesthetic  or  cutaneous  sensation  and  not  to  sound  at  all. 

Watson’s  experiments  also  contribute  data  in  the  comparative 
field  bearing  upon  this  problem  of  backward  association.  He 
tested  white  rats  for  their  ability  to  learn  a  maze  backward  after 

13  Carr,  H.  A.,  and  Freeman,  A.  S.,  “Time  Relationships  in  the  Formation 
of  Association,”  Psychol.  Rev.,  Vol.  XXVI,  p.  465. 


12 


JOSEPH  U.  YARBROUGH 


having  learned  it  in  the  forward  direction.  These  data  are  not 
considered  conclusive,  for  all  who  have  trained  rats  know  that 
considerable  retracing  is  done  while  mastering  a  maze. 

Summary,  i)  The  majority  of  the  experimental  data  re¬ 
viewed  favors  the  simultaneous  mode  of  presentation.  2)  In 
regard  to  the  time  interval,  Bergstrom  is  convinced  that  with  the 
increase  in  the  length  of  the  unfilled  time  interval  the  amount 
learned  increases.  Froeberg  is  of  the  opinion  that  an  increase  in 
the  length  of  the  time  interval  increases  the  difficulty.  3)  All 
investigators  have  found  a  tendency  toward  backward  association 
— a  tendency  varying  in  strength,  but  seldom  equal  to  and  never 
stronger  than  the  forward  tendency. 


III. 

Apparatus  and  Method 


The  ground  plan  of  the  problem  box  used  in  this  experiment 
is  shown  in  Fig.  i.  The  outside  dimensions  are  89"  x  30".  The 
box  is  constructed  of  soft  pine,  and  is  6"  deep.  All  runways 


0 

c 

X 

y 

0 

F 

Di 

0,  Di 

is! 


Ground  Plan  of  Apparatus 

Fig.  1 

and  doors  are  4"  wide  and  5"  deep.  The  doors  are  made  of  very 
thin  metal  and  are  so  hung  that  they  may  be  opened  and  closed 
without  appreciable  noise.  Especially  is  this  true  of  doors  Di 
and  D 2  which  are  controlled  by  rods  R  and  Ri.  The  entire  sur¬ 
face  of  the  box  is  painted  black.  The  food-box  F  is  16"  x  20" 
and  is  the  only  portion  of  the  box  not  covered  with  glass.  The 
entire  floor  of  the  maze  in  front  of  the  food  compartment,  with 
the  exception  of  the  long  runway  from  the  center  door  D,  to  the 
point  O,  is  laid  with  No.  20  copper  wire  at  T/\"  intervals.  By 
pressing  a  contact  key  a  current  from  three  dry  cells  is  sent 
through  an  induction  coil  which  sends  an  induced  current  over 
the  entire  system  of  wire  on  the  floor  of  the  maze.  The  strength 


14 


JOSEPH  U.  YARBROUGH 


of  the  current  passed  over  these  wires  is  so  controlled  that  the 
animal  never  becomes  frightened  by  a  severe  shock. 

This  box  was  developed  from  the  one  used  by  Carr  and  Free¬ 
man1  in  their  experiments.  The  following  improvements  were 
made  in  the  construction  of  the  present  box :  i )  The  glass  cover 
was  extended  so  that  the  animal  is  not  required  to  come  out  in  the 
open  until  it  enters  the  food-box.  2)  There  is  but  one  door  to 
the  food-box.  3)  The  source  of  the  stimuli  is  above  the  glass 
cover,  making  contact  with  the  animal  impossible.  Further¬ 
more,  it  was  necessary  to  increase  the  box  in  length  and  number 
of  compartments  to  secure  sufficient  time  between  the  point 
of  choice  and  the  food-box  for  the  presentation  of  the  two 
stimuli  with  an  interval  of  six  seconds  between  them.  The 
animals  seldom  made  the  distance  in  so  short  a  time,  but  in  their 
most  rapid  responses  the  entire  distance  was  traversed.  The 
additional  apparatus  used  in  presenting  the  pair  of  stimuli  in 
certain  temporal  relations  are  rather  tediously  described  below. 

About  18"  above  the  center  of  the  box  a  light  of  very  low 
candle  power  is  suspended.  The  light  from  this  source  is  just 
sufficient  to  enable  the  experimenter  to  see  the  animal  move 
through  the  maze.  Similarly,  an  electric  buzzer  is  suspended 
10 "  above  the  glass  cover  of  the  middle  portion  of  the  box  and 
connected  with  the  batteries  and  the  timing  apparatus  as  is  in¬ 
dicated  in  Fig.  2.  The  timing  apparatus  is,  in  turn,  attached  to 
a  Zimmerman  kymograph  by  screws  E  and  E1  in  a  way  shown 
very  well  in  Fig.  3..  The  revolving  disc  upon  which  the  arm  A 
rests  is  in  direct  connection  with  the  shaft  of  the  kymograph, 
which  turns  it  around  the  dial  at  any  speed  desired.  The  dial  is 
graduated  so  as  to  read  seconds,  half  seconds,  and  quarter  sec¬ 
onds,  when  the  kymograph  is  adjusted  at  the  rate  of  one  revolu¬ 
tion  for  every  15  seconds.  Each  of  the  numerals,  therefore, 
represents  the  number  of  seconds  necessary  for  the  arm  A  to 
move  from  the  zero  point  to  the  respective  numeral  as  it  appears 
on  the  dial.  The  contact  B  may  be  moved  along  the  dial  and 
firmly  set  at  any  point  by  the  thumb  screw.  The  arm  A  is  not 

1  Carr,  H.  A.,  and  Freeman,  A.  S.,  “Time  Relationships  of  Association,” 
Psychol.  Rev.,  Vol.  XXVI,  p.  465. 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


IS 


fastened  to  the  shaft  of  the  kymograph,  but  is  carried  along  by 
friction  on  the  large  revolving  disc.  If  the  circuit  is  closed  and 
the  electro-magnet  pulls  the  lever  C  into  place,  the  arm  A  is  held 
in  the  position  shown  in  the  drawing.  The  strength  of  the 
magnet  is  great  enough  to  overcome  the  friction  on  the  revolving 
disc. 

By  pressing  the  contact  key  the  circuit  of  the  electro-magnet 
is  broken,  and  is  closed  on  the  electric  buzzer.  This  presssure 
not  only  gives  the  sound  stimulus,  but  it  also  releases  lever  C, 
allowing  arm  A  to  be  carried  along  with  the  revolving  disc  and 
to  pass  over  contact  B,  which  closes  the  circuit  through  the  in¬ 
duction  coil  and  the  system  of  wires  on  the  floor  of  the  box. 
When  the  contact  key  is  released  the  current  again  flows  through 
the  electro-magnet,  attracting  lever  C,  which  keeps  the  arm  A 
from  moving  when  it  again  reaches  that  point.  Thus  the  ap¬ 
paratus  automatically  completes  the  process,  and  after  15  seconds 
it  stands  ready  for  a  repetition  at  the  will  of  the  experimenter. 
The  friction  contacts  at  D  and  B  are  SO'  constructed  that  the 
current  is  broken  at  the  end  of  one-half  second.  Each  stimulus 
is,  therefore,  given  for  this  period  of  time.  By  this  apparatus 
it  is  possible  very  accurately  to  present  two  stimuli  in  a  certain 
definite  temporal  relation  to  each  other. 

With  certain  groups  of  animals  light  was  used  as  one  of  the 
two  terms  to  be  associated.  The  new  apparatus  added  for  pre- 


i6 


JOSEPH  U.  YARBROUGH 


senting  the  light  may  be  seen  in  Fig.  3.  Directly  above  the  long 
central  runway  and  in  contact  with  the  frame  of  the  glass  cover 
of  the  box  a  piece  of  soft  pine  1"  x  2"  -  4'  is  attached,  and  to 
it  is  affixed  two  lamps  connected  in  parallel.  The  first  lamp  is  18" 
from  the  end  of  the  box;  the  second  is  22"  nearer  the  food-box. 
National  Mazda  100  Watt  lamps  are  connected  on  a  115  Volt 
circuit.  I11  order  to  direct  the  light  upon  the  zigzag  pathways 
a  reflector  is  suspended  directly  over  the  lights  and  so  adjusted 
that  the  light  falls  on  these  runways  only.  With  proper  electric 
connections  the  timing  apparatus  enables  us  to  present  sound  and 
light  in  a  certain  known  temporal  relation,  just  as  the  buzzer 
and  pain  were  previously  presented. 

The  method  of  procedure  may  be  described  as  follows :  The 
animal  to  be  tested  is  put  through  the  door  D  from  the  food-box 
F  into  the  central  runway  C.  He  is  required  to  go  up  to  point 
O  and  turn  either  to  the  left  or  to  the  right  and  return  through 
the  zigzag  pathway  X  or  Y,  through  the  door  Di  or  D2,  and 
through  D3  into  the  food-box  F  where  he  is  given  food.  It  is 
of  little  interest  in  these  experiments  whether  the  animal  turns 
to  the  one  side  or  the  other,  or  to  one  side  altogether,  for  the 
problem  is  in  no  way  affected  by  the  alley  chosen  or  by  position 
habit.  The  alley  chosen  in  no  way  determines  what  is  to  follow. 
Seven  times  out  of  ten  trials  the  animal  is  given  an  electric 
shock  in  the  return  zigzag  runway;  no  shock  is  given  in  the 
three  remaining  trials.  This  shock  may  be  given  at  any  point 
along  the  runway,  lasts  for  a  period  of  one-half  second,  and  is 
given  only  once  during  the  trial.  The  problem  here  is  to  learn 
to  turn  around  for  pain  or  to  go  on  in  case  no  pain  is  given. 
If  pain  is  given  and  the  animal  does  not  turn  around  but  goes 
on  toward  the  food-box  he  finds  the  door  at  the  end  of  the  alley 
closed.  Thus,  for  example,  if  the  door  is  D2  he  must,  to  gain 
entrance  to  the  food  box,  run  back  through  the  zigzag  alley  Y, 
pass  the  point  O,  and  approach  through  the  alley  X  and  the  door 
Di.  When  no  pain  is  given  the  door  of  the  alley  selected  is 
always  open  and  the  animal  enters  the  food-box  direct.  The 
problem  here  may,  therefore,  be  thought  of  as  an  association 


Perspective  of  Apparatus 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


1 7 


between  a  closed  door  and  an  electric  shock,  and  farther  on  in 
this  report  it  will  often  be  referred  to  as  learning  the  negative 
response  to  pain. 

From  the  above  conditions  it  will  be  noted  that  no  stimuli 
are  given  in  30  per  cent  of  each  series  of  trials.  This,  together 
with  the  fact  that  chance  and  position  habits  are  of  so  little 
significance  in  the  solution  of  the  problem,  led  us  to  hold  a  rather 
rigid  standard  as  to  the  number  of  trials  and  the  percentage  of 
correct  reactions  to  be  required  of  all  animals.  No  animal  was, 
therefore,  considered  to  have  mastered  the  problem  until  he  had 
made  90  per  cent  correct  responses  on  the  last  100  trials.  All 
animals  used  in  the  experiments  reported  below  were  first  re¬ 
quired  to  learn  this  negative  response  to  pain.  When  this  was 
accomplished  an  electric  buzzer  was  introduced,  by  means  of  the 
timing  apparatus,  in  a  certain  definite  temporal  relation  to  the 
electric  shock,  pain.  The  animals  were  divided  into  groups  and 
each  group  was  set  to  work  upon  the  problem  of  transferring 
this  negative  response  from  pain  over  to  buzzer,  but  each  group 
worked  with  a  different  time  interval.  To  some  groups  the 
stimuli  (buzzer  and  pain)  were  presented  in  a  forward  direction; 
to  others,  in  a  backward — i.e.,  some  groups  received  the  buzzer 
followed  by  pain,  others,  pain  followed  by  buzzer.  In  the  test 
series  the  order  was  always  buzzer  followed  by  pain.  The  prob¬ 
lem  in  each  group  is  to  learn  to  respond  to  the  sound  stimulus 
as  they  have  been  responding  to  pain.  It  is  a  transfer  of  re¬ 
sponse  from  pain  to  sound  over  certain  given  time  intervals. 
Continuous  presentations  and  one,  two,  four,  and  six  second 
intervals  were  studied.  In  the  work  with  the  first  of  the  above 
groups  special  test  series  of  10  trials  each  were  given  after  each 
20  trials  to  determine  to  what  extent  the  transfer  had  been  made. 
In  the  remaining  groups  the  time  interval  was  sufficiently  large 
to  determine  this  without  these  special  tests.  The  standard  for 
mastery  in  each  of  these  groups  is  the  same  as  before,  90  per 
cent  on  the  last  100  trials. 

As  the  animals  completed  this  problem  they  were  regrouped 
and  set  to  work  on  a  third  problem,  but  in  the  regrouping  no 


is 


JOSEPH  U.  YARBROUGH 


animal  was  changed  from  a  group  where  the  stimuli  were  pre¬ 
sented  in  the  backward  to  a  group  where  they  were  presented  in 
the  forward  direction  or  vice  versa.  The  situation  really  re¬ 
mained  the  same  with  the  exception  of  the  changed  stimuli — i.e., 
the  substitution  of  the  response  is  now  from  the  auditory  stimulus 
to  light  instead  of  from  pain  to  sound  as  before.  The  same 
time  intervals  were  studied  as  before.  As  in  the  previous 
experiment,  it  was  necessary  to  enter  the  special  series  of 
test  trials  only  in  the  study  of  the  first  interval  to  determine  the 
extent  to  which  the  transfer  had  been  made  at  any  given  time. 
The  same  standard  is  required  here  as  in  other  problems. 


IV. 

Experimental  Results 

i.  The  Influence  of  the  Time  Interval  in  Successive  Associa¬ 
tion  upon  the  Rate  of  Learning. — The  experiments  bearing  di¬ 
rectly  upon  this  problem  may  be  divided  into  two  sets :  A.  Those 
in  which  the  association  was  learned  in  the  forward  direction; 
B.  Those  in  which  the  association  was  learned  in  the  backward 
direction.  Groups  I  to  XII  were  tested  in  the  first  set  and  Groups 
XIII  and  XIV  in  the  second.  All  the  animals  were  of  about  the 
same  age  and  health  conditions.  Twenty  trials  were  given  each 
day.  To  have  mastered  the  problem  the  animal  must  have  made 
not  less  than  90  per  cent  of  correct  responses  over  a  period  of 
five  consecutive  days,  i.e 90  per  cent  on  the  last  100  trials. 

A.  Learning  the  Association  in  the  Forward  Direction. — As¬ 
sociations  were  made  between  stimuli  from  different  sense  fields. 
Two  pairs  of  stimuli  were  used:  a)  auditory  (buzzer)  and  pain 
(electric  shock),  and  b)  visual  (light)  and  auditory.  The 
auditory-pain  experiments  are  presented  first. 

a)  Association  of  buzzer  with  electric  shock. — In  order  to  se¬ 
cure  comparative  data  upon  the  influence  of  the  time  interval 
between  the  presentation  of  buzzer  and  the  presentation  of  pain, 
the  animals,  upon  mastering  the  problem  of  negative  response  to 
pain,1  were  divided  into  groups,  and  each  group  was  set  to  work 
to  transfer  this  negative  response  from  pain  over  to  the  buzzer. 
The  sound  was  always  presented  before  pain  but  in  a  different 
temporal  relation  to  pain  in  each  of  the  groups.  Other  factors 
remained  constant  for  all  of  the  groups.  In  addition  to  present¬ 
ing  the  stimuli  in  immediate  succession,  i.e.,  in  temporal  contact, 
presentations  at  intervals  of  one,  two,  four,  and  six  seconds  were 
made.  In  all,  38  animals  were  used. 

1 )  Continuous  presentation. — Group  II  was  trained  on  buzzer 
followed  by  the  pain  immediately.  The  results  of  the  work  of 
this  group  are  summarized  in  Table  II. 


1  See  division  3  of  this  section. 


20 


JOSEPH  U.  YARBROUGH 


Animal 

TABLE  II 

Trials 

%  Correct  on 

25 

120 

9i 

26 

120 

90 

27 

140 

92 

28 

120 

91 

29 

120 

90 

50 

100 

93 

Average 

120 

91.: 

A  set  of  test  trials  was  interpolated  after  each  20  trials  of  the 
regular  series  in  order  to  measure  the  gradual  growth  of  the  asso¬ 
ciation.  The  average  number  of  trials  necessary  to  master  this 
problem  was  120,  but  a  study  of  the  curve  in  Fig.  4  and  the 
records  of  the  individuals  of  the  group  show  that  after  60  trials 
some  of  the  group  held  their  record  to  and  above  the  90  per  cent 
mark,  and  that  after  80  trials  no  animal  fell  below  this  point. 

2)  One-second  Interval. — Group  III  was  used  in  these  tests. 
The  buzzer  was  presented  one  second  before  pain,  to  which  they 
had  previously  learned  to  respond  negatively.  If  the  old  re¬ 
sponse  to  pain  was  transferred  to  the  sound  stimulus,  and  the 
animals  turned  around  for  it,  no  pain  was  given.  But  if  they 
failed  to  turn  around  to  the  buzzer  and  instead  continued  toward 
the  food-box,  the  pain  was  given  at  the  end  of  one  second. 
Results  are  given  in  Table  III.  It  was  unnecessary  to  give  the 


TABLE  III 


Animal 

Trials 

%  on  Last  100 

I 

118 

96 

12 

131 

9i 

22 

128 

9i 

24 

126 

92 

35 

148 

90 

37 

120 

87 

Average 

128.5 

91. 1 

special  test  series  in  this  group  in  order  to  determine  the  gradual 
growth  of  the  association,  for  if,  in  the  regular  training  series, 
the  animals  failed  to  turn  around  before  pain  was  given,  the 
response  was  recorded  an  error:  if  they  turned  at  the  sound  no 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


21 


pain  was  given.  These  regular  tests,  therefore,  show  the  gradual 
growth  of  the  association  if  properly  analyzed.  The  analysis 
of  Table  III  is  given  in  the  curve  presented  in  Fig.  4.  Each 
point  of  this  curve  was  determined  by  the  average  per  cent  of 
correct  responses  for  the  group  in  20  trials  of  the  regular  train¬ 
ing  series.  The  growth  of  the  association,  as  determined  by  the 
percentage  of  correct  responses,  is  recorded  for  each  20  trials 
during  the  entire  learning  period. 

The  30  per  cent  of  trials  in  which  no  stimuli  were  given  proved 
to  be  an  excellent  control  for  determining  whether  or  not  the 
animals  had  developed  the  habit  of  turning  each  time  in  the  first 
alley  selected,  or  were  turning  only  when  the  sound  was  encoun¬ 
tered.  Without  an  exception  they  continued  to  the  food-box 
during  these  trials.  Very  often  they  would  proceed  slowly  and 
manifest  hesitancy,  much  as  the  human  does  during  a  period  of 
expectancy  and  uncertainty,  but  in  no  case  did  they  turn  around. 
The  following  conclusion  is  warranted :  The  turning  response 
was  transferred  from  pain  to  buzzer  rather  than  to  some  other 
factor;  128.5  Vials  were  necessary  to  perfect  this  transfer. 

3)  Two-second  interval. — The  two  stimuli  were  presented  to 
Group  IV  with  an  interval  of  two  seconds.  Again  the  special 
tests  were  omitted  and  the  rate  of  of  learning  measured  in  the 
regular  trials.  Tabulated  results  for  the  group  are  to  be  found 
in  Table  IV.  With  the  exception  of  Nos.  10  and  39  the  indi¬ 
vidual  differences  are  small.  With  No.  10  the  association  began 
early  in  the  period,  but  it  was  very  difficult  for  him  to  hold  the 
percentage  of  correct  response  over  a  period  of  five  days.  Num- 

TABLE  IV 


Animal 

Trials 

%  on  Last  100 

10 

316 

92 

11 

210 

9i 

21 

166 

92 

23 

211 

90 

38 

203 

93 

39 

269 

90 

Average 

229 

91.2 

22 


JOSEPH  U.  YARBROUGH 


ber  39  was  very  slow  in  beginning  the  transfer,  but  when  once 
started  the  percentage  increased  normally. 

The  gradual  growth  of  the  association  in  this  group  is  obvious 
from  the  curve  in  Fig.  4.  Each  point  of  this  curve  represents 
the  average  for  the  group  in  percentage  of  correct  response  for 
20  trials  of  the  regular  learning  series.  Although  we  had  used 
the  30  per  cent  of  undisturbed  trials  throughout  these  experi¬ 
ments,  at  the  end  of  the  work  each  animal  was  given  a  special 
series  of  ten  test  trials  in  which  pain  was  withheld.  In  these  tests 
the  response  was  unaffected.  The  group  required  an  average  of 
229  trials  to  reach  the  standard  imposed. 

4)  Four-second  Interval. — With  Group  V  the  sound  was  pre¬ 
sented  four  seconds  before  pain.  Seven  animals  were  used  in 
the  group,  and  Table  V  gives  their  data.  There  are  no  marked 
individual  differences,  a  gradual  increase  in  correct  responses 
being  maintained  by  each  animal,  as  is  graphically  shown  in  the 


learning  curve  in 

Fig-  5- 

TABLE  V 

Animal 

Trials 

%  on  Last  100 

4 

236 

94 

13 

265 

92 

16 

240 

9i 

19 

187 

91 

40 

228 

93 

4i 

216 

94 

52 

269 

91 

Average 

236 

92.4 

After  the  work  was  completed  the  special  set  of  test  trials  was 
given  with  no  effect  upon  the  per  cent  of  correct  responses.  The 
average  number  of  trials  necessary  for  mastery  was  236. 

5)  Six-second  Interval. — Group  VI  made  the  association  over 
an  interval  of  six  seconds.  Table  VI  shows  the  number  of  trials 
necessary  to  make  the  association  and  the  standard  of  correct 
reaction  reached.  From  the  table  and  the  learning  curve  in  Fig. 
5  it  is  possible  to  appreciate  the  general  disturbance  during  the 
first  60  to  100  trials,  as  well  as  the  gradual  growth  of  the  asso¬ 
ciation.  During  this  first  period  of  learning  the  animals  not  only 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


23 


ignored  the  buzzer,  but  also  the  negative  response  to  pain  often 
broke  down. 


TABLE  VI 

Animal 

Trials 

%  on  Last  100 

43 

229 

92 

45 

226 

9i 

46 

217 

9i 

47 

224 

90 

48 

274 

90 

49 

280 

93 

51 

288 

89 

Average  248 . 3 

90.9 

In  other  words,  if 

the  animal  failed  to 

turn  for  the  buzzer 

he  also  often  failed 

to  turn  for  the  pain 

and  continued  his 

course  toward  the  food-box  until  he  encounted  the  closed  door. 

This  same  thing  happened  in  all  the  groups  but  not  so  frequently 

as  in  this  one.  This 

common  period  of  confusion  prehaps  ac- 

counts  for  the  larger  number  of  trials  necessary  for  this  group 

to  master  the  association. 

Summary. — The  results  for  these  five  groups  are  brought  to- 

gether  in  Table  VII. 

From  these  results 

the  following  con- 

TABLE  VII 

Group 

Time  Interval 

Trials 

II 

0  seconds 

120 

III 

1  “ 

128.5 

IV 

2  “ 

229 

V 

4  “ 

236 

VI 

6  “ 

248.3 

Average  192.3 


elusions  are  warranted:  1.  Measured  in  terms  of  the  number 
of  trials  necessary  to  learn  the  association,  the  difficulty  of  mak¬ 
ing  the  transfer  of  response  does  not  increase  uniformly  for  the 
increase  in  the  length  of  the  time  interval  between  the  two  stimuli 
presented.  2.  There  is  a  disproportionate  increase  in  dif¬ 
ficulty  when  the  interval  is  increased  from  one  to  two  seconds. 
3.  There  is  practically  no  increase  in  difficulty  when  the  interval 


24 


JOSEPH  U.  YARBROUGH 


Figure  5 

Curves  2,  3  and  4  (Fig.  4)  are  the  learning  curves  for  the  Groups  II, 
III  and  IV,  respectively.  Curves  4,  5  and  6  (Fig.  5)  are  the  learning  curves 
for  Groups  IV,  V,  and  VI. 

is  increased  from  two  to  four  seconds.  4.  The  six-second  inter¬ 
val  is  more  difficult  than  the  four-second,  but  the  increase  in 
difficulty  is  not  so  great  as  from  the  one-second  to  the  two-second 
interval. 

b)  Association  of  Light  with  Sound.' — When  the  animal  had 
made  the  association  of  sound  with  pain,  the  pain  had  in  reality 
dropped  out,  and  the  animal  responded  to  the  buzzer  whenever 
presented.  To  demonstrate  the  accuracy  of  this  statement  a 
special  set  of  test  trials  was  given  at  the  end  of  the  learning 
period  in  which  pain  was  not  given  at  all,  and  the  percentage  of 
correct  response  was  not  affected.  It  was  possible,  therefore,  to 


1 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


25 


introduce  another  stimulus  affecting  still  another  sense  at  certain 
definite  time  intervals  before  the  buzzer,  just  as  the  buzzer  had 
been  entered  before  pain  in  the  groups  previously  reported. 
Light  was  selected  for  this  new  stimulus.  By  properly  connect¬ 
ing  the  timing  apparatus  in  the  light  circuit  as  described  above, 
it  was  possible  to  present  the  two  stimuli  (sound  and  light)  with 
a  known  time  interval  between  them.  The  animals  were  re¬ 
grouped  and  five  new  groups  were  set  to  work  on  learning  to 
transfer  their  motor  response  from  sound  over  to  light.  With 
the  exception  of  the  time  interval,  conditions  were  constant  for 
all  the  groups.  As  before,  experiments  were  made  with  con¬ 
tinuous  presentation  and  with  one,  two,  four,  and  six  second  in¬ 
tervals.  The  90  per  cent  standard  was  again  adopted.  In  all, 
24  animals  were  used  on  this  problem. 

1 )  Continuous  presentation. — Here  the  problem  was  the  asso¬ 
ciation  of  light  with  sound  when  presented  in  immediate  suc¬ 
cession.  The  data  for  this  group  are  given  in  Table  VIII.  One 

TABLE  VIII 


Animal 

Trials 

%  on  Last  100 

48 

100 

96 

49 

100 

89 

50 

100 

9i 

51 

100 

90 

Average 

100 

91.5 

hundred  trials  were  given  to  each  animal,  but  the  test  trials  show 
that  the  association  was  complete  at  the  end  of  60  trials.  The 
curve  in  Fig.  6  shows  a  period  of  fluctuation  in  correct  response 
between  the  regular  series  and  the  test  trials.  After  60  trials  in 
the  regular  series  the  curve  remains  almost  uniform  at  90  and 
above.  This  uniformity  was  reached  in  Group  II  at  the  end  of 
40  trials.  A  comparison  of  this  curve  with  that  of  Group  II, 
Fig.  4,  indicates  that  the  light  and  the  buzzer  offered  a  problem 
of  practically  equal  difficulty  with  the  sound-pain  association  in 
the  first  part  of  the  period,  but  the  transfer  to  light  was  more 
readily  made.  From  a  study  of  the  individual  records  in  the 
two  groups  it  was  found  that  under  these  conditions  when  ani- 


26 


JOSEPH  U.  YARBROUGH 


mals  once  learn  to  respond  to  light  they  hold  a  more  uniformly 
high  record  than  when  responding  to  sound.  On  the  basis  of 
the  number  of  trials  necessary  to  make  the  association  it  was 
easier  for  the  animals  to  associate  light  with  sound  than  sound 
with  pain,  when  the  two  stimuli  were  presented  continuously. 

2)  One-second  interval. — The  animals  in  Group  IX  made  the 
light-buzzer  association  over  a  one-second  interval.  They  had 
just  transferred  the  negative  response  from  pain  to  buzzer,  and 
the  new  problem  called  for  this  response  to  be  transferred  to 
light,  the  latter  being  presented  one  second  before  the  former. 
When  the  animal  turned  to  the  light,  the  buzzer  was  not  sounded. 
Failure  to  respond  negatively  to  light  was  recorded  an  error. 
Table  IX  gives  the  data  for  this  group.  No  special  test  trials 


TABLE  IX 


Animal 

Trials 

%  on  Last  100 

37 

160 

94 

45 

140 

92 

46 

140 

94 

47 

140 

93 

Average 

145 

93*2 

were  given,  the  results  of  the  regular  training  series  being  suf¬ 
ficient  to  show  the  rate  of  learning  the  association.  The  per¬ 
centage  is  very  low  at  the  beginning  of  the  learning  period  and 
increases  rapidly,  giving  a  positive  accelerating  curve.  When 
compared  with  the  learning  curve  for  Group  III,  where  all  con¬ 
ditions  are  the  same  except  the  stimuli,  two  marked  differences 
are  found :  1 )  The  percentage  of  correct  response  in  the  first  20 
trials  is  considerably  lower  for  Group  IX;  2)  no  learning  is 
indicated  by  Group  IX  until  after  40  trials,  while  learning  appears 
to  have  begun  immediately  with  the  other  group.  Under  these 
conditions,  145  trials  were  necessary  to  transfer  the  negative 
response  from  the  sound  to  the  light. 

3)  Two-second  interval. — To  Group  X  the  stimuli  were  pre¬ 
sented  with  an  interval  of  two  seconds  between  them.  The 
animals  dropped  low  in  efficiency  in  the  first  series,  but  when 
learning  began  the  curve  rose  as  rapidly  as  that  for  Group  IX. 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


27 


See  data  in  Table  X  and  the  learning  curve  in  Fig.  6.  The  prob¬ 
lem  for  Group  X  differed  from  that  for  Group  IV  only  in  the 
matter  of  stimuli  used,  but  there  is  a  marked  difference  between 


TABLE  X 


Animal 

Trials 

%  on  Last  100 

13 

160 

93 

16 

180 

90 

19 

160 

92 

35 

180 

93 

Average 

170 

92 

the  curves  for  the  two  groups.  In  the  Group  IV  curve  is  shown 
a  period  of  no  learning,  followed  by  a  brief  plateau  and  a  second 
period  of  gradual  growth,  which  brings  the  record  up  to  the 
standard.  In  contrast,  the  Group  X  curve  shows  a  shorter  period 
of  no  learning,  followed  by  rapid  progress.  There  are  two  pos¬ 
sible  explanations  for  this  dissimilarity.  First,  it  may  be  due 
to  the  previous  training  the  animals  in  Group  X  had  had  in  the 
problem  of  pain-sound  transfer.  Second,  the  pain-sound  trans¬ 
fer  may  be  more  difficult  than  the  sound-light  transfer. 

4)  Four-second  Interval. — The  animals  in  Group  XI  were 
trained  to  associate  light  with  the  buzzer  when  presented  with 


erval  of 

four  seconds.  Table  XI 

TABLE  XI 

records  the  results. 

• 

Animal 

Trials 

%  on  Last  100 

4 

160 

92 

23 

140 

93 

38 

160 

9i 

39 

160 

90 

Average 


155 


91.5 


From  these  data  and  the  curve  in  Fig.  7  one  may  see  the  rate 
of  learning  the  association  and  the  number  of  trials  necessary 
to  master  the  problem.  There  are  no  marked  differences  between 
the  data  of  this  group  and  those  of  Group  X  above.  In  each 
case  there  is  a  complete  breakdown  in  the  early  part  of  the  period 
followed  by  a  period  of  rapid  learning.  With  this  group  there 


28 


JOSEPH  U.  YARBROUGH 


Curves  8,  9,  and  10  (Fig.  6)  ^re  the  learning  curves  for  Groups  VIII,  IX, 
and  X,  respectively.  Curves  10,  11  and  12  (Fig.  7)  are  the  curves  for 
Groups,  X,  XI  and  XII. 


appears  to  be  no  increase  in  difficulty  with  an  increase  of  the 
time  interval  from  two  to  four  seconds.  But  when  compared 
with  the  data  of  Group  V  in  which  sound  and  pain  were  pre¬ 
sented  in  the  same  temporal  relation,  two  points  of  difference 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


29 


appear.  First,  the  breakdown  in  the  beginning  of  the  learning 
period  is  more  marked  in  the  latter.  A  percentage  of  45  was 
made  in  this  group,  while  in  Group  V  an  efficiency  of  62  per 
cent  was  maintained.  Second,  the  learning  curve  rises  much 
more  rapidly  for  Group  XI.  The  percentage  of  increase  rose  from 
45  to  95  in  four  series  of  trials,  while  in  Group  V  the  increase 
for  the  corresponding  period  was  from  67  to  74  per  cent.  Seven 
days  were  required  for  Group  XI  to  reach  the  90  per  cent  mark 
of  correct  response.  Previous  training  is  an  explanatory  factor, 
but  indications  are  that  this  difference  is  largely  due  to  difference 
in  the  stimuli  used.  The  data  thus  far  gathered  indicate  that, 
other  things  being  equal,  it  is  more  difficult  for  white  rats  of  the 
type  used  in  these  experiments  to  associate  sound  with  pain  than 
light  with  sound. 

5)  Six-second  Interval. — Group  XII  learned  the  association 
when  light  was  presented  six  seconds  before  sound.  In  Table 
XII  is  given  the  data  for  this  group.  The  association  over  this 
time  interval  required  more  trials  than  did  that  for  any  other 
group,  but  the  difference  is  not  great  enough  to  be  of  significance. 
The  curve  in  Fig.  7  will  assist  in  showing  the  rate  of  learning. 
In  form  it  follows  rather  closely  the  curve  for  Group  XI — low 
at  first,  followed  by  a  period  of  rapid  learning. 


Animal 

TABLE  XII 

Trials 

%  on  Last  100 

1 

160 

9i 

10 

200 

94 

11 

180 

92 

12 

180 

95 

Average 

180 

92.5 

The  effect  of  stimulus  difference  is  shown  by  comparing  this 
curve  with  that  of  Group  VI.  The  curve  for  the  former  group 
begins  somewhat  higher,  and  instead  of  rising  higher  it  first 
tends  downward  and  later  ascends,  gradually  reaching  the  90 
per  cent  mark  after  180  trials;  the  curve  for  the  latter  exhibits  a 
very  brief  period  of  no  learning,  followed  by  very  rapid  progress. 

Summary:  To  enable  the  reader  to  see  at  a  glance  the  results 


30 


JOSEPH  U.  YARBROUGH 


obtained  in  each  of  these  five  groups  the  following  table  was 
constructed.  From  these  data  and  the  curves  presented  the  fol- 

TABLE  XIII 


Group 

Time 

Interval 

Trials 

VIII 

o  seconds 

IOO 

IX 

I 

U 

145 

X 

2 

a 

I/O 

XI 

4 

a 

155 

XII 

6 

a 

180 

Average 

150 

lowing  conclusions  were  reached:  Judged  on  the  basis  of  the 
number  of  trials  necessary  to  make  the  association,  the  difficulty 
of  the  problem  of  transfer  of  response  from  light  to  sound 
appears  not  to  have  increased  proportionately  with  the  increase 
in  time  interval  between  the  presentation  of  the  two  stimuli. 
Practically  the  same  increase  in  difficulty  is  manifest  in  going 
from  continuous  presentation  to  the  one-second  interval  as  in  go¬ 
ing  from  the  one  to  the  two-second  interval.  The  four-second 
interval,  on  the  other  hand,  seems  easier  to  master  than  the  two- 
second  but  less  easy  than  the  one-second.  The  learning  curves 
start  low,  go  through  a  short  period  of  no  learning,  and  then 
rise  rapidly  in  each  group. 

B.  Learning  the  Association  in  the  Backward  Direction. — In 
reading  the  literature  on  the  phenomena  of  association  one  is 
lead  to  the  belief  that  psychologists  generally  hold  to  the  con¬ 
clusion  that  recall  is  predominantly  in  the  forward  direction. 
Some  experimental  data,  however,  indicate  that  an  association 
will  function  backward  as  well  as  forward.  Freeman  found 
that  “the  formation  of  an  association  between  a  stimulus  and  a 
motor  response  by  animals  is  exceedingly  difficult  and  perhaps 
impossible  when  the  stimulus  is  presented  after  the  act  has  oc¬ 
curred.  ”  Since  our  method  of  procedure  and  experimental  con¬ 
ditions  were  so  different  it  was  decided  to  include  an  examination 
of  this  point. 

Two  groups  of  animals  (Groups  XIII  and  XIV)  were  used 
in  these  experiments,  the  problem  being  the  transfer  of  the  pre- 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


3i 


viously  learned  negative  response  from  pain  over  to  the  auditory 
stimulus  (electric  buzzer).  The  buzzer  was  given  in  each  case 
after  the  pain  had  been  given:  to  Group  XIII  it  was  presented 
in  immediate  succession;  to  Group  XIV,  with  an  interval  of  one 
second  separating  it  from  pain.  Thus,  in  the  latter  case  the  two 
factors  were  presented  in  the  backward  order  with  a  time  inter¬ 
val  of  one  second  between  them,  i.e.,  the  sound  stimulus  was  pre¬ 
sented  after  the  motor  response  to  pain  had  been  initiated  and  in 
some  cases  completed.  All  the  animals  were  given,  in  addition 
to  the  regular  training  series,  a  series  of  test  trials.  The  object 
of  the  training  series  was  to  establish  an  effective  association 
between  the  turning  response  to  pain  and  the  auditory  stimulus; 
the  object  of  the  test  trials  was  to  determine  the  degree  to  which 
an  effective  association  had  been  formed.  In  these  tests  the  pro¬ 
cedure  was  the  same  as  in  the  training  series  except  that  no  pain 
was  given.  If  the  negative  response  had  been  transferred  to  the 
auditory  stimulus,  they  should  turn  and  retrace  the  path  when 
the  buzzer  was  sounded.  A  correct  response  consisted  of  an 
immediate  turn  when  the  auditory  stimulus  was  presented,  or 
of  traversing  the  whole  length  of  the  path  into  the  food-box 
in  the  30  per  cent  of  trials  in  which  no  stimulus  was  given. 
Progress  was  measured  by  the  increasing  percentage  of  correct 
response  during  the  test  trials.  For  in  each  group  the  established 
negative  response  to  pain  prevented  the  animals  from  responding 
to  sound  during  the  training  series.  They  would  most  often 
have  turned  around  or  be  turning  around  when  the  auditory 
stimulus  was  given.  It  is  obvious,  therefore,  that  the  new  stim¬ 
ulus  which  was  to  be  associated  with  the  turning  response  oc¬ 
curred  after  the  act  of  turning  had  been  initiated.  A  correct 
response  in  the  regular  training  series  is  the  same  as  it  was  before 
the  auditory  stimulus  was  introduced,  i.e.,  an  immediate  turn 
when  the  pain  stimulus  was  presented,  or  traversing  the  whole 
length  of  the  path  into  the  food-box  in  the  30  per  cent  of  trials 
in  which  no  stimulus  was  given. 

The  number  of  animals  used,  the  number  of  training  trials 
necessary  to  make  the  association,  and  the  percentage  of  correct 


?2 


JOSEPH  U.  YARBROUGH 


responses  for  Group  XIII  are  given  in  Table  XIV.  In  the  test 
series  only  one  term  of  the  pair  to  be  associated  was  presented. 
These  trials  would,  therefore,  not  be  added  to  the  regular  train¬ 
ing  series  in  order  to  secure  the  number  of  trials  necessary  to 


TABLE  XIV 


Rat 

Training  Trials 

Test  Trials 

%  on  Last 

53 

140 

60 

90 

54 

120 

50 

92.5 

55 

140 

60 

94 

56 

120 

50 

91 

Average  130 

55 

91 .8 

make  the  association.  Their  effect  would  really  be  expected  to 
hinder  the  rate  of  learning,  since  each  time  they  took  the  place 
of  a  regular  day's  work,  thus  making  a  break  in  the  regular 
training  series.  Since  each  animal  completed  his  work  on  the  test 
series,  the  percentage  of  right  response  on  the  last  ioo  trials,  pre¬ 
sented  in  the  last  column  on  the  table,  includes  four  series  (40) 
of  test  trials  and  three  (60)  of  trainnig  trials.  It  was  planned 
to  use  only  the  test  trials  as  a  basis  for  this  percentage,  but  since 
each  animal  had  held  a  percentage  of  90  or  above  for  a  period 
of  seven  days  we  were  satisfied  that  the  transfer  of  response 
was  completely  made. 

From  Graph  13  in  Fig.  8  the  rate  at  which  the  association  was 
made  can  easily  be  observed.  The  values  in  this  curve  represent 
the  percentages  of  correct  response  in  the  test  trials  given  at  the 
end  of  the  first  40  trials  and  following  each  series  of  20  trials 
thereafter.  The  test  series  was  not  introduced  in  these  groups 
until  after  the  first  40  trials,  because  it  was  assumed  that  not 
until  after  then  would  the  transfer  of  response  be  sufficiently 
made  to  make  it  worth  while  to  interrupt  the  regular  training 
series.  Since  the  correct  response  in  the  30  per  cent  of  trials  in 
which  no  stimulus  was  given  had  become  thorougly  learned  in 
the  previous  training,  mistakes  in  these  trials  seldom  occurred. 
Consequently,  the  percentage  records  of  this  group  may  be  ex¬ 
pected  to  begin  at  30  and  increase  to  90  and  above  as  the  motor 
response  is  transferred  to  the  auditory  stimulus.  Since  the  test 


rate  of  learning  in  the  white  rat 


33 


trials  interpolated  after  the  first  40  training  trials  show  79  per 
cent  of  correct  response,  it  is  safe  to  infer  that  considerable 
transfer  was  made  during  these  40  trials.  The  association  was 
readily  made;  with,  in  fact,  but  little  more  difficulty  than  was 
experienced  when  the  two  stimuli  were  presented  continuously  in 
the  forward  direction.  This  small  difference  may  be  due  to 
chance  or  to  the  fact  that  a  smaller  group  was  used  in  these  last 
experiments. 

When  the  animals  of  Group  XIII  had  learned  the  association 
of  negative  motor  response  to  sound,  light  was  presented  in  the 
same  temporal  relation  to  sound  as  sound  had  been  to  pain  in 
the  previous  training,  i.e.,  in  immediate  succession.  By  this 
means  it  was  hoped  to  get  some  data  on  the  relative  merit  of  the 
different  stimuli.  Table  XV  depicts  the  animals  used,  the  num¬ 
ber  of  trials  necessary  to  master  the  problem,  and  the  percentage 
of  right  response  on  the  last  100  trials.  A  comparison  of  this 


TABLE  XV 


Rat 

53 

54 

55 

56 


Training  Trials 


Test  Trials 


°/o  on  Last  100 


120 

50 

92 

IOO 

40 

90 

120 

50 

92 

120 

50 

91 

Average  1 15 

47-5 

9i-5 

Table  XIV  and  of  Graph 

13L  in  Fig. 

8  (the  curves 

for  the  present  group)  with  Graph  13  of  the  same  figure  shows 
no  marked  difference  between  the  two — so  little,  in  fact,  that  we 
are  not  warranted  in  making  the  statement  that  it  is  easier  to  as¬ 
sociate  a  motor  response  with  light  than  with  sound  when  all 
other  conditions  are  constant.  It  is  preferable  to  account  for 
this  small  difference  on  the  basis  of  the  effect  of  transfer  of 
learning. 


In  Group  XIV  where  the  sound  was  presented  one  second 
after  the  pain  (in  most  cases  after  the  motor  response  to  pain) 
the  conditions  were  in  all  other  respects  the  same  as  those  for 
Group  XIII.  Under  these  conditions  we  hoped  to  secure  data 
showing  the  relative  difficulty  in  making  the  association  in  the 


34 


JOSEPH  U.  YARBROUGH 


backward  direction  when  the  two  members  to  be  associated  are 
presented  continuously  and  when  they  are  presented  with  an 
interval  of  one  second  between  them.  The  data  for  this  group 
are  given  in  Table  XVI.  The  last  column  of  this  table  is  mis- 


TABLE  XVI 


Rat 

Training  Trials 

Test  Trials 

%  on  I 

57 

320 

70 

73 

58 

340 

80 

77 

59 

380 

90 

68 

60 

320 

70 

76 

Average  340 

77-5 

73 

leading  unless  it  is  understood  that  this  last  ioo  trials  include 
four  series  (80)  of  the  regular  training  trials  and  only  two  series 
(20)  of  the  test  trials.  This  is  accounted  for  by  the  fact  that 
the  test  trials  were  interpolated  after  the  first  60  regular  trials 
and  after  each  20  subsequent  trials.  This  was  done  because  of 
the  apparent  slow  rate  of  making  the  association  under  these  con¬ 
ditions.  The  rate  of  increase  in  the  strength  of  the  association 
can  be  observed  more  accurately  by  reference  to  Graph  14  in 

Fig.  8.  This  curve  is  constructed  on  the  basis  of  percentages  of 

« 

correct  response  on  the  test  trials  only.  Although  all  the  animals 
made  occasional  correct  responses  to  the  sound  stimulus  during 
the  test  trials  no  real  learning  is  observable  at  the  end  of  380 
training  trials.  It  is  to  be  regretted  that  the  work  of  this  group 
was  not  continued,  but  sickness  caused  the  experiments  to  close 
at  this  point.  Yet,  several  features  of  the  results  deserve  com¬ 
ment:  1)  Measured  in  terms  of  the  number  of  trials  necessary 
to  make  the  association  function  in  the  backward  direction,  it  is 
much  more  difficult  to  form  the  association  when  the  terms  pre¬ 
sented  are  separated  by  one  second  than  when  they  are  presented 
continuously.  The  animals  in  the  group  to  which  the  stimuli 
were  presented  continuously  learned  the  association  in  an  average 
of  130  trials,  while  those  in  the  former  group  showed  no  im¬ 
provement  at  the  end  of  340  trials.  With  more  training  the 
animals  of  the  group  might  have  learned  the  association,  but  we 
have  no  evidence  that  they  would  have  done  so.  2)  Judged  upon 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


35 


the  same  basis,  it  is  more  difficult  to  learn  to  associate  a  motor 
response  with  an  auditory  stimulus  when  the  latter  is  presented 
one  second  after  the  former  has  occurred  than  when  it  is  pre¬ 
sented  one  second  prior  to  the  initiation  of  the  motor  response. 
Under  the  latter  condition  Group  III  learned  the  association  on  an 
average  of  128.5  trials,  while  under  the  former  conditions  Group 
XIV  failed  to  make  the  association,  although  they  had  an  average 
of  340  trials.  3)  When  the  stimuli  are  presented  in  immediate 
succession  it  is  more  difficult  for  the  association  to  function  in 
the  backward  than  in  the  forward  direction.  To  account  for 
the  comparative  ease  with  which  the  animals  made  the  associa¬ 
tion  in  the  backward  direction  under  these  conditions,  one  may 
assume  that  the  new  stimulus  was  not  associated  with  the  old 
stimulus.  This  response  and  the  new  stimulus  were  experienced 
simultaneously,  and  association  is  due  to  this  simultaneity. 

Summary  : 

1.  Association  in  the  backward  direction  is  only  slightly  more 
difficult  than  in  the  forward  direction  when  the  terms  to  be  asso¬ 
ciated  are  presented  in  immediate  succession.  This  small  differ¬ 
ence  is  by  no  means  conclusive. 

2.  It  is  much  more  difficult  to  learn  an  association  in  the 
backward  direction  when  the  terms  are  separated  by  one  second 
than  when  they  are  presented  in  immediate  succession. 

3.  It  is  more  difficult  to  associate  a  motor  response  with  an 
auditory  stimulus  when  the  latter  is  presented  after  the  former 
has  occurred  than  when  it  is  presented  before  the  initiation  of 
the  motor  response. 

4.  The  data  found  do  not  warrant  the  statement  that  it  is 
easier  to  associate  a  motor  response  with  light  than  with  sound 
when  the  stimuli  are  presented  in  immediate  succession. 

2.  Simultaneous  and  Successive  Presentation. — The  present 
investigation  was  not  undertaken  primarily  for  the  purpose  of 
gathering  experimental  data  relating  to  the  well  known  contro¬ 
versy  between  simultaneous  and  successive  association  advo¬ 
cates,  but  the  condition  of  our  problem  made  it  profitable  to 
do  so.  Experimental  data  thus  far  reported  are  divided  on  this 


36 


JOSEPH  U.  YARBROUGH 


In  Fig.  8  curves  2  and  3  are  reproduced  from  Fig.  4;  curve  13  is  the 
learning  curve  for  Group  XIII  on  sound,  and  curve  13L  is  for  the  same 
group  on  light.  Curve  14  represents  the  work  of  Group  XIV.  In  Fig.  9 
curve  15  is  the  composite  curve  for  simultaneous  groups  (Groups  I  and 
VII)  ;  curve  16  is  the  composite  curve  for  successive  groups  (Groups  II 
and  VIII). 

point,  but  the  better  controlled  experiments  report  data  indicat¬ 
ing  the  superiority  of  the  simultaneous  mode.  The  difference  in 
materials  used,  experimental  conditions,  and  methods  of  pro¬ 
cedure,  no  doubt  account  for  the  conflicting  results.  Our  data 
support  the  superiority  of  simultaneous  presentation.  Almost 
all  results  thus  far  reported  have  come  from  investigations  with 
human  subjects,  a  fact  which  gives  our  data  the  more  significance. 

1)  Simultaneous  Mode  of  Presentation. — Two  groups  of  ani¬ 
mals  learned  to  make  the  association  when  the  two  stimuli  were 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


37 


presented  simultaneously.  With  the  one,  electric  shock  and 
buzzer  were  used ;  with  the  other,  electric  buzzer  and  light.  The 
previously  discussed  test  trials  were  interpolated  after  each  series 
of  20  training  trials  in  order  to  check  the  rate  of  learning.  These 
tests  were  not  made  with  Nos.  2  and  9.  These  continued  on  the 
regular  learning  series  until  they  had  had  100  trials  before  the 
test  series  was  entered.  These  animals  do  not  appear  to  have  had 
any  advantage  over  the  other  animals  of  this  group  where  the 
special  tests  were  entered  earlier.  These  special  tests  were  not 
expected  to  assist  in  learning  the  association,  but  they  do  not 
appear  to  have  hindered  it. 

When  the  pain-buzzer  group  (Group  I)  had  learned  to  respond 
negatively  to  the  first  stimulus,  pain,  buzzer  was  sounded  simul¬ 
taneously  with  pain,  and  the  problem  was  to  transfer  the  negative 
response  from  pain  over  to  buzzer.  In  Table  XIV  the  number 
of  trials  necessary  to  master  the  problem  and  the  strength  of  the 
association  are  given.  The  number  of  test  trials  given  is  not 
shown  in  this  table,  but  is  used  in  constructing  the  curve  in  Fig.  9. 

TABLE  XIV 


Animal 

Training  Trials 

%  on  Last  100 

2 

120 

9i 

9 

100 

96 

14 

100 

9i 

3i 

100 

9i 

32 

120 

90 

33 

140 

92 

Average  113.3 

91.6 

No  marked  individual  differences  appear.  Although  the  average 
number  of  trials  necessary  to  learn  the  association  is  113.3,  in 
each  case  where  the  test  trials  were  given  the  animal  had  so  com¬ 
pletely  made  the  transfer  that  at  the  end  of  60  trials  he  made 
90  per  cent  on  the  test  trials.  However,  because  90  per  cent 
on  the  last  100  trials  had  been  adopted  as  a  standard,  each  of  the 
animals  was  given  additional  trials  on  the  training  series. 

The  four  animals  of  Group  VII  had  not  only  learned  the 
negative  response  to  pain,  but  had  transferred  this  response  to 


38 


JOSEPH  U.  YARBROUGH 


the  sound  stimulus.  After  the  habitual  response  to  the  buzzer 
had  been  built  up  an  electric  light  was  presented  simultaneously 
with  the  buzzer.  It  was  hoped  by  this  means  to  determine  the 
rate  and  number  of  trials  necessary  to  transfer  this  response 
from  sound  over  to  the  simultaneously  experienced  light.  During 
the  first  series  of  20  trials  only  a  slight  disturbance  was  shown, 
the  average  percentage  of  correct  response  for  the  group  being  86. 
The  animals  continued  to  respond  to  the  sound  but  in  the  presence 
of  light,  a  rather  disturbing  factor.  The  special  series  of  10  test 
trials  broke  down  the  response.  Only  24  per  cent  above  the 
minimum  was  made  by  the  group,  with  two  animals  falling  to 
10  per  cent  above.  Table  XV  contains  the  individual  records. 

TABLE  XV 

Training  Trials  %  on  Last  100 

100  93 

100  96 

100  91 

100  92 

Average  100  93 

At  the  end  of  100  trials  each  animal  had  made  the  association. 
The  transfer  again  was  very  rapid,  and  only  a  comparatively 
small  number  of  trials  were  necessary  to  make  the  association. 

The  learning  curves  for  these  two  groups,  given  in  Fig.  9 
depict  the  rapid  growth  of  the  association.  The  construction  is 
based  on  the  per  cent  of  right  responses  in  both  the  regular  series 
and  the  special  test  trials.  The  first  and  each  alternate  point 
thereafter  is  determined  by  the  average  per  cent  of  right  response 
for  the  group  in  20  regular  training  trials.  The  other  points  are 
the  average  per  cent  of  efficiency  for  the  group  in  10  special 
test  trials.  From  the  curve  it  is  seen  that  Group  VII  formed  the 
association  by  the  end  of  40  trials,  for  at  no  time  after  this  did 
the  percentage  of  the  group  fall  below  90. 

When  these  two  groups  are  compared  there  are  two  points  to 
be  noted.  1)  In  both  groups  the  percentage  of  right  response  on 
the  first  20  learning  curves  is  practically  the  same.  In  these  trials 
the  animals  were  reacting  to  an  old  stimulus  but  in  the  presence 


Animal 

49 

41 

43 

52 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


39 


of  a  new  one;  namely,  sound  for  Group  I  and  light  for  Group 
VII.  The  two  stimuli  seemed  to  offer  about  equal  disturbing 
effect  at  this  point  in  the  learning.  2)  When  the  first  special 
test  series  was  given  a  complete  breakdown  occurred  in  Group 
VII  while  Group  I  only  partially  broke  down.  The  drop  in  per¬ 
centage  of  right  response  was  from  85  to  55  in  the  former,  and 
from  84  to  62  in  the  latter.  This  breakdown  was  only  temporary, 
for  in  the  next  special  series  the  percentage  rose  above  that  of 
Group  I.  Light  seems  to  be  more  difficult  in  the  beginning,  but 
measured  on  the  basis  of  the  required  number  of  trials  in  the 
two  groups  there  is  little  or  no  difference. 

2)  Continuous  Mode  of  Presentation. — The  work  of  these 
two  groups  (Groups  II  and  VIII)  was  reported  with  the  other 
groups  on  successive  association,  but  in  the  table  below  the  re¬ 
sults  are  reproduced  in  order  to  get  a  quick  comparison  of  the 
two  modes  of  presentation.  The  curves  presented  in  Fig.  9 

TABLE  XVI 


Simultaneous  Trials 

Group  I .  II3-3 

Group  VII .  100 

Continuous  Trials 

Group  II .  120 

Group  VIII .  100 


show  the  different  rate  of  learning  the  association  by  the  two 
groups. 

With  these  results  before  us  we  are  able  to  observe  the  follow¬ 
ing  facts:  1)  Associations  are  possible  with  either  the  simul¬ 
taneous  or  the  successive  mode  of  presentation.  2)  Our  data 
indicate  the  superiority  of  the  simultaneous  mode  of  presentation, 
in  that  under  these  conditions  the  association  is  more  readily 
made — i.e.,  fewer  trials  are  necessary  to  establish  it  than  when 
the  stimuli  are  presented  in  succession.  3)  The  theory  that 
associations  are  possible  only  when  the  terms  to  be  associated 
are  presented  simultaneously  is  not  supported  by  our  results. 

3.  Learning  the  Negative  Response  to  Pain.1 — All  the  animals 

1  For  method  of  procedure,  one  may  with  profit  re-examine  the  last  few 
pages  of  Section  III  above. 


40 


JOSEPH  U.  YARBROUGH 


used  in  the  experiments  of  this  monograph  learned  the  negative 
response  to  pain  before  beginning  work  on  their  respective  prob¬ 
lems.  In  training  the  animals  to  make  this  response  two  mazes 
were  used.  With  one  group  of  23  animals  the  maze  shown  in 
Fig.  1  was  used;  and  the  maze  described  by  Carr  and  Freeman 
in  the  report  of  their  studies2  was  selected  for  the  training  of  the 
latter  group.  The  results  of  the  former  group  are  presented 
first. 


The  results  for  the 

23 

animals  trained  on 

the  first  mentioned 

box  are  given  in  Table  XVII.  The  first  column  gives  the  animal; 
the  second,  the  number  of  trials  necessary  to  establish  the  desired 
response,  and  the  third  the  actual  number  of  turns  made  by  each 
animal  before  he  attained  the  adopted  standard.  Since  90  per 
cent  of  correct  responses  must  be  made  within  the  last  100  trials, 

it  is  obvious  that  few 

turns  were  made  until 

near  the  end  of  the 

TABLE  XVII 

Animals 

Trials  No.  of  Turns  to  Pain 

I 

820 

112 

2 

770 

98 

4 

1015 

157 

9 

745 

107 

10 

684 

99 

11 

890 

135 

12 

901 

151 

13 

1024 

136 

14 

1009 

11 3 

16 

1079 

1 18 

19 

753 

132 

21 

7i5 

119 

22 

914 

128 

23 

803 

143 

24 

926 

99 

>  25 

1045 

123 

26 

1119 

102 

27 

ii34 

106 

28 

937 

IOI 

29 

1159 

160 

3i 

1376 

124 

32 

979 

109 

33 

1251 

116 

Average 

960.6 

121 .2 

2  Op.  cit.,  p.  4 66. 

RATE  OF  LEARNING  IN  THE  WHITE  RAT 


41 


training.  The  number  of  trials  necessary  to  master  this  problem 
varied  from  684  for  No.  10  to  1376  for  No.  31,  while  the 
average  for  the  group  was  960.6.  Since  no  pain  was  given  in 
30  per  cent  of  the  trials  these  animals  on  the  average  encountered 
pain  672  times  before  learning  to  make  the  desired  response. 

Because  of  the  large  number  of  trials  required  and  the  amount 
of  time  lost  in  the  long  zigzag  runways  of  this  box,  it  was  thought 
expedient  to  use  a  simpler  box  in  training  the  remaining  animals. 
The  apparatus  used  by  Carr  and  Freeman  was  selected  with  the 
hope  that  time  could  be  saved  each  day  by  its  use.  Future  ex¬ 
periments  required  that  the  animal  be  able  to  hold  the  standard 
of  correct  response  in  the  box  presented  in  this  paper  and  not  in 
the  simpler  one.  It  was  necessary,  therefore,  that  both  apparatus 
be  learned,  a  necessity  which  made  it  seem  probable  that  the 
group  working  in  the  new  box  would  require  more  trials  than  the 
other  group.  In  order  to  avoid  any  delay  in  making  the  transfer 
of  correct  response  from  one  box  to  the  other,  the  animals  were 
each  day  given  one  run  in  the  long  maze  after  having  completed 
the  day's  work  in  the  short  one.  In  this  way  the  two  mazes 
were  learned  together:  the  first  being  partially  learned,  then  the 
second  partially  learned,  and  finally  the  first  and  then  the  second 
completely  learned.  In  the  light  of  the  findings  of  Wiltbank1 
this  was  thought  to  be  the  best  possible  method.  Table  XVIII 
gives  the  number  of  animals,  the  number  of  trials,  and  the  num¬ 
ber  of  turns  made  by  each  animal  before  reaching  the  required 
standard  of  efficiency.  The  range  of  trials  required  is  from  394 
to  1196;  the  average  for  the  group  of  15  rats  is  694.4.  Since 
in  30  per  cent  of  these  trials  no  pain  was  given,  the  animals 
received  pain  486  times  before  learning  to  respond  to  it  nega¬ 
tively.  Not  only  did  this  method  save  time  in  giving  the  series 
from  day  to  day,  but  on  the  average  it  reduced  the  number  of 
presentations  of  pain  from  672  (the  number  required  for  the 
long-maze  groups)  to  468 — a  saving  of  204.  There  are  at  least 
three  possible  explanations  of  this  difference:  1)  The  increased 
complexity  of  the  longer  box  increased  the  difficulty  of  the  prob- 

1  Wiltbank,  R.  T.,  “Transfer  of  Training  in  White  Rat  Upon  Various 
Series  of  Mazes/’  Beb.  Mono.,  Vol.  IV,  1919. 


42 


JOSEPH  U.  YARBROUGH 


TABLE  XVIII 


Animals 

Trials 

No.  of  Turns 

35 

772 

103 

37 

1196 

102 

38 

787 

115 

39 

817 

108 

40 

429 

98 

4i 

840 

94 

43 

578 

9i 

45 

651 

104 

46 

679 

93 

47 

394 

88 

48 

652 

94 

49 

585 

96 

50 

654 

9i 

5i 

614 

98 

52 

768 

107 

Average 

694.4 

98.8 

lem.  The  larger  number  of  compartments  greatly  increased  the 
number  of  turns  and  the  distance  to  be  traversed  in  ecah  trial, 
thereby  causing  confusion.  The  records  show  a  small  number 
of  instances  where  an  animal,  after  having  run  the  maze  for 
several  hundred  times,  became  lost  and  ran  back  and  forth 
through  three  to  five  adjacent  compartments  several  times  before 
he  proceeded  to  the  food-box.  2)  With  the  short  box  the  trials 
were  more  frequent  than  with  the  long  one.  It  took  less  than 
half  the  time  to  run  the  smaller  maze;  thus,  the  trials  could  be 
given  twice  as  frequently.  3)  The  getting  of  food  was  delayed 
by  the  long  runways  of  the  larger  box.  The  influence  of  delay¬ 
ing  food,  the  apparent  motive  for  the  animal’s  effort,  is  not 
known,  but  we  are  of  the  opinion  that  it  was  an  important  fac¬ 
tor  with  our  animals. 

Although  the  number  of  trials  was  reduced  when  the  shorter 
box  was  used  there  was  really  no  difference  in  the  rate  of  learn¬ 
ing  between  the  two  groups  after  learning  once  began.  A  study 
of  the  learning  curves  in  Fig.  10  confirms  the  accuracy  of  this 
statement.  In  constructing  these  curves  only  the  last  500  trials 
for  each  animal  were  used.  This  was  done  for  two  reasons: 
1)  All  observable  learning  is  included  in  this  period.  2)  It  per- 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


43 


Fig.  io 

Curve  15  in  Fig.  10  represents  the  rate  of  learning  in  the  last  500  trials 
for  groups  of  animals  trained  in  our  maze :  Curve  16,  the  rate  for  those 
trained  in  the  Freeman  maze. 

r 

mits  a  comparison  of  the  rate  of  learning  at  the  exact  time  learn¬ 
ing  was  in  progress  in  the  individual  cases.  This  is  not  possible 
in  either  group  if  the  curve  is  plotted  on  the  basis  of  trials  made 
from  the  beginning  of  the  problem,  for  some  completed  the 
training  before  others  showed  signs  of  improvement.  In  each 
group  there  is  a  period  of  no  apparent  learning  followed  by  a 
short  period  of  rapid  learning. 

The  large  number  of  animals  in  each  of  these  groups  and  the 
almost  uniform  difference  between  the  number  of  trials  required 
to  master  the  problem  are  conclusive  evidence  that  the  negative 
response  to  pain  can  be  more  easily  learned  in  a  simple  maze 
than  in  a  more  complex  one,  and  that  the  rate  of  learning  is 
approximately  the  same  from  the  time  obvious  learning  first  ap¬ 
pears  until  the  problem  is  mastered. 


V. 


Interpretation  of  Results 

Now  that  the  experimental  results  have  been  presented  there 
remains  the  important  task  of  ascertaining,  as  far  as  we  can 
do  so,  the  relative  scope  and  value  of  the  different  factors  in¬ 
volved.  In  making  this  interpretation  and  evaluation  the  dif¬ 
ferent  theories  that  have  been  advanced  by  the  various  investi¬ 
gators  to  account  for  the  experimental  results  previously  reported 
will  be  discussed.  The  following  questions  set  forth  the  chief 
points  of  discussion:  i)  How  can  association  be  accounted  for 
when  the  two  terms  to  be  associated  are  presented  with  a  certain 
time  interval  between  them?  2)  How  can  association  be  made 
when  the  two  terms  are  presented  in  the  reverse  order?  3)  Is 
the  simultaneous  mode  of  presentation  superior  to  the  successive? 
These  questions  are  considered  below  in  the  order  given. 

1.  The  nature  of  the  connection  established  between  the  two 
members  of  a  pair  of  external  stimuli,  the  first  member  of  which 
is  presented  for  a  definite  time  and  is  succeeded  by  the  second 
after  a  definite  interval,  is  the  problem  to  the  solution  of  which 
the  numerous  experimental  investigations,  beginning  with  Eb- 
binghaus  and  continuing  through  the  past  25  years,  have  aimed 
to  contribute.  Thus  far,  three  theories  have  been  advanced  for 
the  explanation  of  this  phenomenon.  1)  One  may  assume  the 
survival  of  a  memory  image  of  the  first  member  of  a  pair  of 
stimuli  during  the  time  interval  which  separated  it  from  the 
second  stimuli.  The  second  term  of  the  association,  then,  coin¬ 
cides  in  time  with  the  “gradually-dying-away”  memory  image 
of  the  first,  and  it  is  due  to  this  simultaneity  that  the  association 
is  formed.  2)  The  connection  between  the  stimuli  may  be  me¬ 
diated  by  some  third  factor  which  is  not  only  common  to  the  two 
stimuli,  but  is  also  present  during  the  interval  which  separates 
them.  For  example,  a  series  of  syllables  may  be  more  success¬ 
fully  memorized  by  associating  each  syllable  with  its  position  in 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


45 


a  temporal  sequence.  The  continued  running  activity  of  the 
animal  and  the  external  environment  were  possible  third  factors 
in  our  experiments.  If  the  running  activity,  which  was  almost 
always  present,  did  serve  such  a  mediating  function  no  increase 
in  difficulty  in  forming  the  association  over  different  time  inter¬ 
vals  should  have  been  evidenced.  Nor  can  this  variation  among 
the  different  groups  be  accounted  for  on  the  basis  of  the  external 
environment,  for  it  remained  constant.  3)  The  third  theory 
assumes  that  the  two  terms  may  be  associated  directly,  i.e.,  when 
the  first  member  of  a  pair  of  stimuli  ceases  the  actual  nervous 
excitation  is  not  finished  but  continues  for  a  time,  gradually 
diminishing  in  strength.  The  second  member  coincides  with  the 
diminishing  phase  of  the  preceding  one,  and  the  association  is 
due  to  this  simultaneity  of  the  two  experiences.  Although  the 
external  stimuli  are  presented  in  succession  the  internal  ex¬ 
periences  are  in  a  large  measure  coexistent. 

In  the  light  of  our  data,  this  “gradually  diminishing  nervous 
excitation”  continues  for  at  least  six  seconds.  This  after-phase 
may  account  for  the  distinct,  though  disproportionate,  increase 
in  difficulty  in  making  the  association  over  larger  and  larger 
intervals.  If  this  difficulty  in  making  the  association  is  a  measure 
of  the  waning  strength  of  the  after-phase  of  the  first  term,  our 
data  warrant  the  statement  that  little  or  no  loss  in  strength  is 
experienced  during  the  first  second,  but  by  the  end  of  two  seconds 
a  marked  weakening  is  observed.  For  longer  intervals  the  diminu¬ 
tion  is  gradual  with  no  marked  breaks  observable.  This  is  in 
agreement  with  Froeberg's  finding  that  association  can  be  formed 
over  an  interval  of  at  least  five  seconds. 

Although  the  hypothesis  of  direct  association  seems  to  be  the 
best  explanation  of  successive  association,  the  following  pos¬ 
sible  objections  to  its  acceptance  should  be  pointed  out:  a) 
When  human  subjects  are  used  it  may  be  assumed  that  all 
memory  and  thought  connection  are  not  wholly  excluded.  These 
connections  are  many,  and  some  are  very  difficult  to  overcome, 
b)  Where  activities  are  interpolated  to  destroy  the  memory  image 
of  the  preceding  term  it  may  be  assumed  that  these  activities 


/ 


46 


JOSEPH  U.  YARBROUGH 


themselves  mediate  the  association.  However,  in  our  experi¬ 
ments  with  animals  these  objections  are  not  valid,  for  few  be¬ 
lieve  that  white  rats  master  problems  by  the  use  of  memory 
images  and  concepts,  c)  A  third  and  perhaps  a  more  vital  ob¬ 
jection  to  this  theory  has  been  made  by  Carr;  namely,  that  while 
one  can  easily  conceive  of  the  persistence  of  a  neural  activity 
for  a  short  interval  of  time  on  the  principle  of  inertia,  yet  the 
assumption  of  an  indefinite  continuance  rather  taxes  one’s  cre¬ 
dulity.  For  this  reason  a  fourth  type  of  explanation  is  per¬ 
haps  the  most  acceptable.  4)  This,  the  last  theory  advanced, 
explains  the  connection  formed  between  the  two  stimuli  in  terms 
of  the  decreased  resistance  of  the  motor  center.  The  connection 
is  mediated  by  so  decreasing  the  resistance  of  the  motor  center 
that  the  one  term  will  become  the  adequate  motor  outlet  for  the 
nervous  impulse  aroused  by  the  other.  Such  a  conception  is 
based  upon  the  well  known  and  generally  accepted  theory  of 
synaptic  resistance.  The  two  theories  have  in  common  two 
assumptions :  1 )  Each  sensory  impulse  tends  to  follow  that 

motor  path  which  for  the  time  ofifers  the  minimum  of  resistance. 
2)  The  resistance  of  any  center  is  decreased  by  an  increase  in 
metabolism  in  that  center.  If  the  resistance  of  one  motor  center 
has  by  continued  practice  been  so  reduced  that  its  readiness  for 
response  is  much  greater  than  that  of  any  other  motor  outlet, 
there  are  no  reasons  why  an  effective  association  cannot  be 
formed  over  a  considerable  interval  of  time. 

Since  there  was  no  observable  third  factor  mediating  the  asso¬ 
ciation  in  our  work  and  since  survival  of  the  memory  image 
most  likely  is  not  a  factor  in  the  behavior  of  the  white  rat,  there 
remain  but  two  possible  explanations  for  our  results :  namely, 
either  the  theory  of  the  “akoluthic”  phase  or  the  reduced  resist¬ 
ance  of  the  motor  center. 

2.  Our  discussion  has  thus  far  been  concerned  with  the 
phenomenon  of  successive  association  when  the  two  members  to 
be  associated  were  presented  in  a  certain  definite  temporal  rela¬ 
tion.  We  shall  now  examine  successive  association  when  formed 
under  the  same  conditions  except  that  the  order  of  the  presenta- 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


47 


tion  of  the  terms  is  reversed.  Under  these  conditions  the  con¬ 
nection  between  the  terms  may  be  explained  upon  the  same  hypo¬ 
thesis  as  in  the  experiments  in  the  forward  direction.  For  there 
is  no  reason  for  the  assumption  that,  since  the  animal  has  made 
a  certain  motor  response  to  the  first  term,  the  neural  excitation 
produced  by  this  term  suddenly  ceases.  On  the  contrary,  if  the 
stimulus  gets  sufficient  attention  to  call  out  this  motor  response 
its  neural  effect  might  be  expected  to  diminish  more  gradually 
than  when  no  reaction  is  made  to  it.  The  assumption  is  made 
here  that  the  “akoluthic”  phase  diminishes  at  a  rate  proportionate 
to  the  intensity  of  the  stimulus,  which,  of  course,  we  have  not 
proved.  Although  our  data  are  not  conclusive  on  this  point 
they  indicate  greatly  increased  difficulty  in  learning  the  associa¬ 
tion  when  the  terms  are  presented  in  the  backward  order.  This 
increase  in  difficulty  may  be  explained  by  factors  of  the  situation 
other  than  the  “akoluthic“  phase — factors  incident  to  the  reversal 
of  the  stimuli,  i)  The  animal  had  previously  been  thoroughly 
trained  to  turn  when  pain  was  presented.  As  this  response  was 
learned  it  was  easy  to  observe  a  change  in  his  behavior  from  a 
careless,  random  run  to  a  slower  movement  which  resembled 
human  behavior  when  in  a  state  of  expectancy.  This  method  of 
approach  made  him  more  responsive  to  any  stimulus  presented  at 
the  time.  He,  therefore,  readily  transferred  the  response  from 
pain  to  sound  when  the  buzzer  was  presented  before  the  motor 
response.  If,  however,  the  buzzer  was  presented  after  the  motor 
response  had  occurred  this  state  of  “expectancy’’  was  absent, 
and  the  transfer  of  response  was  more  difficult.  2)  The  previous 
training  had  taught  the  animals  that  when  once  turned  they  must 
retrace  the  maze  and  select  the  opposite  side  in  order  to  secure 
food.  Without  exception  this  was  true.  Hence,  after  they 
turned  around  for  pain  and  started  on  a  never-failing  road  to  the 
food-box,  it  was  scarcely  probable  that  they  would  proceed  in 
the  same  state  of  “expectancy”  as  before.  Daily  observations 
showed  that  they  did  not.  3)  One  may  assume  that  the  two 
terms  were  associated,  but  that  it  was  more  difficult  for  them  to 
function  in  the  backward  direction.  The  conditions  for  learning 

o 


43 


JOSEPH  U.  YARBROUGH 


here  are  probably  more  difficult  than  in  learning  the  negative 
response  to  pain. 

3.  In  most  of  the  experiments  reported  the  problem  of  suc¬ 
cessive  and  simultaneous  mode  of  presentation  has  been  im¬ 
portant.  In  these,  successive  presentation  has  been  thought  of 
as  immediate  succession  of  the  members  of  the  stimuli.  The  re¬ 
sults  are  divided  as  to  the  better  mode  of  presentation,  but  the 
majority  of  the  data  favor  the  simultaneous  method.  The  writer 
is  of  the  opinion  that  the  difference  in  results  is  best  accounted 
for  by  the  difference  in  the  material  used  in  the  several  experi¬ 
ments  and  difference  in  the  method  of  procedure. 

The  most  important  point  of  difference  in  method  of  pro¬ 
cedure  is  the  attempt  to  secure  an  exposure  time  in  successive 
presentation  equal  in  length  to  that  in  simultaneous  presentation. 
If  each  member  of  the  pair  in  the  successive  series  is  exposed 
for  the  same  length  of  time  as  the  pair  of  simultaneous  stimuli 
the  learning  time  is  twice  as  long  in  the  successive  as  in  the 
simultaneous  presentation.  Wohlgemuth  felt  a  necessity  for 
making  this  exposure  time  equal  in  length  if  comparative  data 
were  to  be  secured.  To  accomplish  this  he  devised  two  methods : 
1 )  The  exposure  time  for  the  simultaneous  pair  was  made  twice 
as  long  as  for  each  member  of  the  successive  pair,  the  number  of 
presentations  remaining  the  same;  2)  The  exposure  time  re¬ 
mained  the  same  while  the  number  of  presentations  were  twice 
as  great  in  the  simultaneous  as  in  the  successive  pairs.  Froeberg 
objects  to  both  of  these  methods.  To  the  first  his  objection  is 
that  it  “implies  that  the  strength  of  an  association  depends  upon 
the  exposure  time  of  the  stimuli  only,  regardless  of  whether  or 
not  the  pair  is  exposed  at  the  same  time,  and  thus  contradicts 
the  theory  which  was  to  be  proved,  namely  that  association  takes 
place  only  while  the  two  experiences  are  present  in  conscious¬ 
ness.”  Of  the  second  method  his  statement  is  that  “it  disregards 
the  effect  of  the  distribution  of  presentations.” 

The  criticism  of  the  second  method  of  equalizing  the  ex¬ 
posure  time  is  pertinent,  but  so  much  cannot  be  said  for  the 
first.  This  first  method  is  good  or  bad,  depending  upon  which 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


49 


phase  of  the  problem  is  under  investigation.  If  the  problem 
attempts  to  determine  the  effect  of  the  exposure  time  this  is  a 
good  method,  but  if  the  aim  is  to  determine  whether  or  not  two 
terms  when  presented  simultaneously  offer  greater  interference 
to  each  other  than  when  presented  successively,  thereby  hin¬ 
dering  the  process  of  learning,  the  method  is  inept.  The  method 
used  in  our  experiments  and  also  by  Froeberg  is  preferable  in 
such  investigations.  By  means  of  it  each  term  is  exposed  for  an 
equal  time,  regardless  of  the  mode  of  presentation.  In  simul¬ 
taneous  presentation  the  exposure  time  of  each  of  the  two  terms 
is  not  only  equal,  but  the  presentations  actually  coincide  in  time. 
In  the  successive  mode  the  exposure  time  of  each  of  the  terms 
is  also  equal,  but  the  presentations  succeed  each  other  in  time. 
It  is  obvious  that  such  a  method  reveals  the  relative  value  of  the 
two  modes  of  presentation  on  the  basis  of  term  interference. 

Our  data  do  not  indicate  term  interference  when  the  two  terms 
are  presented  simultaneously.  On  the  other  hand,  they  indicate 
a  more  rapid  learning  with  the  simultaneous  than  with  the  suc¬ 
cessive  mode.  These  findings  are  in  agreement  with  those  of 
Chamberlain,  Bigham,  Wohlgemuth,  and  Froeberg,  when  the 
latter  used  nonsense  syllables  as  material. 

As  indicated  above,  the  results  have  varied  when  different 
materials  were  used.  Disconnected  words,  associable  words, 
nonsense  syllables,  syllables,  letters,  colors,  and  diagrams  have 
served  as  materials  for  the  experiments  thus  far  reported  in  the 
field  of  human  psychology.  From  a  study  of  all  the  results  ob¬ 
tained  the  following  conclusion,  reached  by  Froeberg,  appears 
to  be  true:  If  the  material  is  such  that  the  pair  of  stimuli  form 
an  organic  whole  or  can  be  attended  as  a  unit,  simultaneous 
presentation  is  preferable;  if  it  is  such  that  the  materials  stand 
as  separate  entities,  or  must  be  attended  to  separately  in  order  to 
be  apprehended,  successive  presentation  is  preferable. 

In  the  field  of  Animal  Psychology  a  certain  motor  response 
and  a  definite  external  stimulus  have  served  as  the  two  terms  to 
be  associated.  After  a  motor  response  to  one  term  is  built  up 
the  second  term  is  presented  in  a  certain  temporal  relation  to 


50 


JOSEPH  U.  YARBROUGH 


the  first — i.e.,  either  before  or  during  or  after  the  motor  response 
has  occurred.  In  the  present  experiments  light  and  buzzer  were 
introduced  in  this  way  in  relation  to  the  motor  response,  and  the 
results  indicate  in  both  instances  the  superiority  of  simultaneous 
presentation.  The  learning  was  more  rapid  when  light  was  used 
than  when  sound,  but  this  fact  is  due  rather  to  transfer  of  train¬ 
ing  than  to  a  difference  in  stimuli. 


VI. 

Summary  and  Conclusion 

1.  The  number  of  trials  necessary  to  learn  the  association 
does  not  increase  proportionately  with  the  increase  of  the  time 
interval  between  the  presentation  of  sound  and  pain. 

2.  Upon  the  basis  of  the  number  of  trials  necessary  to  make 
the  association,  the  difficulty  of  the  problem  steadily  increases 
with  the  increase  of  the  time  interval  between  the  presentation  of 
sound  and  light,  but  this  increase  is  not  proportionate. 

3.  Our  data  indicate  no  marked  difference  in  difficulty  for 
the  continuous  and  one-second  interval  presentations;  likewise, 
no  marked  difference  is  manifest  for  the  intervals  above  two 
seconds.  The  point  of  disproportionate  increase  is  between  one 
and  two  seconds. 

4.  The  excess  in  the  number  of  trials  required  for  the  animals 
to  transfer  a  motor  response  to  sound  over  the  number  required 
to  transfer  this  response  to  light  is  perhaps  due  to  the  sense 
material  used,  yet  the  transfer  of  training  is  an  important  factor 
here. 

5.  Association  in  the  backward  direction  is  very  little,  if  any, 
more  difficult  than  in  the  forward  direction  when  the  continuous 
mode  of  presentation  is  used.  If,  however,  the  two  terms  to  be 
associated  are  presented  with  a  time  interval  of  one  second  be¬ 
tween  them,  the  association  in  the  reverse  direction  is  perhaps 
impossible. 

6.  Under  our  experimental  conditions  and  with  the  sensory 
material  used,  our  data  indicate  a  slight  superiority  of  the  simul¬ 
taneous  mode  of  presentation.  In  order  to  show  the  relation  of 
our  results  to  those  of  earlier  investigators  of  simultaneous  and 
successive  association,  we  may  classify  the  findings  of  the  latter 
upon  the  basis  of  exposure  time  per  term.  Wohlgemuth  (two 
experiments),  Bigham,  and  Chamberlain  used  an  unequal  ex¬ 
posure  time  per  term,  the  inequality  favoring  the  simultaneous 
mode,  and  in  every  instance  they  found  the  simultaneous  mode 


52 


RATE  OF  LEARNING  IN  THE  WHITE  RAT 


to  be  the  better.  In  the  experiments  of  Wohlgemuth  (one  ex¬ 
periment),  Froeberg,  and  Freeman  the  exposure  time  per  term 
was  equal,  and,  except  for  Froeberg’s  work  with  nonsense  syl¬ 
lables,  all  results  favor  succession  as  the  better  mode.  Froeberg 
thinks  his  exception  is  not  a  proof  of  the  superiority  of  the  simul¬ 
taneous  mode,  but  is  rather  to  be  accounted  for  by  the  sense 
material  used.  Although  our  results  indicate  a  slight  superiority 
of  the  simultaneous  mode,  this  is  not  sufficient  to  be  of  great 
significance.  These  data  seem  to  warrant  the  conclusion  that 
when  the  exposure  time  per  term  is  unequal  the  simultaneous 
mode  is  the  better;  when  it  is  equal  the  better  mode  of  presenta¬ 
tion  depends  upon  the  materials  used. 


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